diff --git a/frontend/src/App.jsx b/frontend/src/App.jsx
index 76683e6..e6534ea 100644
--- a/frontend/src/App.jsx
+++ b/frontend/src/App.jsx
@@ -1,3 +1,4 @@
+import { useEffect, useState } from "react";
 import { Navigate, Route, Routes } from "react-router-dom";
 import { Toaster } from "sonner";
 
@@ -13,6 +14,23 @@ import { AuthCallbackPage } from "./pages/AuthCallbackPage";
  * can wrap `<App />` with a `MemoryRouter` if needed.
  */
 export default function App() {
+  const [isMobile, setIsMobile] = useState(false);
+
+  useEffect(() => {
+    const mediaQuery = window.matchMedia("(max-width: 640px)");
+
+    const updateViewport = () => {
+      setIsMobile(mediaQuery.matches);
+    };
+
+    updateViewport();
+    mediaQuery.addEventListener("change", updateViewport);
+
+    return () => {
+      mediaQuery.removeEventListener("change", updateViewport);
+    };
+  }, []);
+
   return (
     <AuthProvider>
       <Routes>
@@ -24,20 +42,24 @@ export default function App() {
       </Routes>
 
       <Toaster
-        position="bottom-right"
+        position={isMobile ? "bottom-center" : "bottom-right"}
         richColors
         closeButton
+        expand
         theme="light"
+        visibleToasts={4}
+        offset={20}
+        mobileOffset={{ bottom: 12, left: 12, right: 12 }}
         toastOptions={{ duration: 4000 }}
         style={{
           /* SUCCESS — ORCID corporate green */
-          '--success-bg': '#EAF3DE',
-          '--success-border': '#C0DD97',
-          '--success-text': '#3B6D11',
+          "--success-bg": "#EAF3DE",
+          "--success-border": "#C0DD97",
+          "--success-text": "#3B6D11",
           /* ERROR — hue-0° mirror of the ORCID green (same saturation & lightness) */
-          '--error-bg': '#F3DDDD',
-          '--error-border': '#DD9797',
-          '--error-text': '#6E1111',
+          "--error-bg": "#F3DDDD",
+          "--error-border": "#DD9797",
+          "--error-text": "#6E1111",
         }}
       />
     </AuthProvider>
diff --git a/frontend/src/components/dashboard/SyncButton.jsx b/frontend/src/components/dashboard/SyncButton.jsx
index bca8420..0a95dd2 100644
--- a/frontend/src/components/dashboard/SyncButton.jsx
+++ b/frontend/src/components/dashboard/SyncButton.jsx
@@ -5,9 +5,15 @@ import { Spinner } from "../ui/Spinner";
  * Primary action button on the dashboard. Swaps icon + colour scheme
  * depending on the sync lifecycle (idle → loading → success flash).
  */
-export function SyncButton({ onClick, status = "idle", className = "" }) {
+export function SyncButton({
+  onClick,
+  status = "idle",
+  disabled = false,
+  className = "",
+}) {
   const isLoading = status === "loading";
   const isSuccess = status === "success";
+  const isDisabled = disabled || isLoading || isSuccess;
 
   const palette = isSuccess
     ? "bg-orcid-green-soft text-orcid-green-text border border-orcid-green-border"
@@ -19,7 +25,7 @@ export function SyncButton({ onClick, status = "idle", className = "" }) {
     <button
       type="button"
       onClick={onClick}
-      disabled={isLoading}
+      disabled={isDisabled}
       className={`inline-flex items-center justify-center gap-2 rounded-lg px-[18px] py-2.5 text-sm font-medium transition-colors disabled:cursor-not-allowed ${palette} ${className}`.trim()}
     >
       {isLoading ? (
diff --git a/frontend/src/index.css b/frontend/src/index.css
index de2e2b1..c155ede 100644
--- a/frontend/src/index.css
+++ b/frontend/src/index.css
@@ -80,3 +80,94 @@ body {
   -webkit-font-smoothing: antialiased;
   -moz-osx-font-smoothing: grayscale;
 }
+
+/* Sonner toaster UX polish (desktop + mobile). */
+[data-sonner-toaster] {
+  --normal-width: min(92vw, 560px);
+  --width: min(92vw, 560px);
+  --border-radius: 16px;
+}
+
+[data-sonner-toaster][data-y-position="top"] {
+  top: max(12px, env(safe-area-inset-top));
+}
+
+[data-sonner-toast] {
+  padding: 14px 16px;
+  border: 1px solid var(--color-surface-border-strong);
+  box-shadow:
+    0 14px 34px rgba(16, 24, 40, 0.14),
+    0 3px 10px rgba(16, 24, 40, 0.08);
+}
+
+[data-sonner-toast] [data-title] {
+  font-size: 1rem;
+  font-weight: 700;
+  line-height: 1.25;
+  letter-spacing: -0.01em;
+}
+
+[data-sonner-toast] [data-description] {
+  margin-top: 4px;
+  font-size: 0.94rem;
+  line-height: 1.45;
+  color: var(--color-ink-secondary);
+}
+
+/* Large hit-area + clear visual state for close button. */
+[data-sonner-toast] [data-close-button] {
+  width: 36px;
+  height: 36px;
+  border-radius: 999px;
+  border: 1px solid var(--color-surface-border-strong);
+  background: #fff;
+  color: var(--color-ink-secondary);
+  box-shadow: 0 1px 2px rgba(16, 24, 40, 0.08);
+}
+
+[data-sonner-toast] [data-close-button]:hover {
+  background: #f8f7f3;
+  color: var(--color-ink-primary);
+}
+
+[data-sonner-toast] [data-close-button]:focus-visible {
+  outline: 2px solid var(--color-brand-accent);
+  outline-offset: 2px;
+}
+
+[data-sonner-toast] [data-button] {
+  min-height: 36px;
+  border-radius: 10px;
+  font-weight: 600;
+}
+
+@media (max-width: 640px) {
+  [data-sonner-toaster] {
+    --normal-width: min(90vw, 380px);
+    --width: min(90vw, 380px);
+  }
+
+  [data-sonner-toast] {
+    padding: 12px 12px 10px;
+  }
+
+  [data-sonner-toast] [data-title] {
+    font-size: 0.95rem;
+    line-height: 1.3;
+  }
+
+  [data-sonner-toast] [data-description] {
+    font-size: 0.9rem;
+    line-height: 1.4;
+  }
+
+  /* 44x44 touch target for thumbs on mobile. */
+  [data-sonner-toast] [data-close-button] {
+    width: 40px;
+    height: 40px;
+  }
+
+  [data-sonner-toast] [data-button] {
+    min-height: 40px;
+  }
+}
diff --git a/frontend/src/pages/DashboardPage.jsx b/frontend/src/pages/DashboardPage.jsx
index b0c60a4..10acd15 100644
--- a/frontend/src/pages/DashboardPage.jsx
+++ b/frontend/src/pages/DashboardPage.jsx
@@ -23,6 +23,9 @@ import {
 import { useAuth } from "../contexts/AuthContext";
 
 const SUCCESS_FLASH_MS = 3000;
+/** Minimum gap between sync requests (protects backend + avoids toast spam). */
+const SYNC_COOLDOWN_MS = 5000;
+const SYNC_TOAST_ID = "researcher-sync";
 
 /**
  * Researcher detail page. Owns:
@@ -52,6 +55,10 @@ export function DashboardPage() {
   const [pubsError, setPubsError] = useState(null);
 
   const [syncStatus, setSyncStatus] = useState("idle"); // idle | loading | success
+  const [syncCooldownActive, setSyncCooldownActive] = useState(false);
+  const syncInFlightRef = useRef(false);
+  const syncCooldownUntilRef = useRef(0);
+  const syncCooldownTimerRef = useRef(null);
   const [exportingFormat, setExportingFormat] = useState(null);
   const [exportDestination, setExportDestination] = useState(
     DEFAULT_EXPORT_DESTINATION,
@@ -110,12 +117,44 @@ export function DashboardPage() {
     return () => ctrl.abort();
   }, [orcid, loadBundle]);
 
+  useEffect(() => {
+    return () => {
+      if (syncCooldownTimerRef.current) {
+        clearTimeout(syncCooldownTimerRef.current);
+      }
+    };
+  }, []);
+
+  const syncDisabled = syncStatus !== "idle" || syncCooldownActive;
+
+  function startSyncCooldown() {
+    syncCooldownUntilRef.current = Date.now() + SYNC_COOLDOWN_MS;
+    setSyncCooldownActive(true);
+    if (syncCooldownTimerRef.current) {
+      clearTimeout(syncCooldownTimerRef.current);
+    }
+    syncCooldownTimerRef.current = setTimeout(() => {
+      setSyncCooldownActive(false);
+      syncCooldownTimerRef.current = null;
+    }, SYNC_COOLDOWN_MS);
+  }
+
   if (!isValidOrcid(orcid)) {
     return <Navigate to="/" replace />;
   }
 
   async function handleSync() {
+    if (
+      syncInFlightRef.current ||
+      syncStatus !== "idle" ||
+      Date.now() < syncCooldownUntilRef.current
+    ) {
+      return;
+    }
+
+    syncInFlightRef.current = true;
     setSyncStatus("loading");
+
     try {
       const bundle = await syncResearcher(orcid);
       setResearcher(bundle.researcher);
@@ -132,6 +171,7 @@ export function DashboardPage() {
       const { newRecords, updatedRecords, totalRecords } = bundle;
       const hasChanges = newRecords > 0 || updatedRecords > 0;
       toast.success("Sincronización completada", {
+        id: SYNC_TOAST_ID,
         description: hasChanges
           ? `${newRecords} nuevas · ${updatedRecords} actualizadas (${totalRecords} total).`
           : "Sin cambios desde la última sincronización.",
@@ -140,8 +180,12 @@ export function DashboardPage() {
     } catch (err) {
       setSyncStatus("idle");
       toast.error("Error al sincronizar con ORCID", {
+        id: SYNC_TOAST_ID,
         description: err?.message ?? "Inténtalo de nuevo más tarde.",
       });
+    } finally {
+      syncInFlightRef.current = false;
+      startSyncCooldown();
     }
   }
 
@@ -227,6 +271,7 @@ export function DashboardPage() {
                   <SyncButton
                     onClick={handleSync}
                     status={syncStatus}
+                    disabled={syncDisabled}
                     className="w-full sm:w-auto"
                   />
                   <ExportDropdown