fix: add onCleanup for setTimeout in frontend pages

Prevents timers from firing after component unmount in Home (delete
confirmation timers), ArticleHistory (clear confirmation timer), and
SynthesisDetail (email success banner timer).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/ArticleHistory.tsx

@@ -2,6 +2,7 @@ import {
   type Component,
   createSignal,
   onMount,
+  onCleanup,
   Show,
   For,
   createEffect,
@@ -56,6 +57,12 @@ const ArticleHistory: Component = () => {
   const [filterSourceType, setFilterSourceType] = createSignal('');
   const [page, setPage] = createSignal(0);
   const [confirming, setConfirming] = createSignal(false);
+  const [confirmTimer, setConfirmTimer] = createSignal<ReturnType<typeof setTimeout> | undefined>();
+
+  onCleanup(() => {
+    const t = confirmTimer();
+    if (t) clearTimeout(t);
+  });
 
   const totalPages = () => Math.max(1, Math.ceil(total() / PAGE_SIZE));
 
@@ -92,7 +99,7 @@ const ArticleHistory: Component = () => {
   const handleClear = async () => {
     if (!confirming()) {
       setConfirming(true);
-      setTimeout(() => setConfirming(false), 3000);
+      setConfirmTimer(setTimeout(() => setConfirming(false), 3000));
       return;
     }
     try {

frontend/src/pages/Home.tsx

@@ -2,6 +2,7 @@ import {
   type Component,
   createSignal,
   onMount,
+  onCleanup,
   Show,
   For,
 } from 'solid-js';
@@ -32,6 +33,11 @@ const Home: Component = () => {
   const [deletingId, setDeletingId] = createSignal<string | null>(null);
   const [deleteTimers, setDeleteTimers] = createSignal<Record<string, ReturnType<typeof setTimeout>>>({});
 
+  onCleanup(() => {
+    const timers = deleteTimers();
+    Object.values(timers).forEach((timer) => clearTimeout(timer));
+  });
+
   onMount(async () => {
     try {
       const data = await synthesesApi.list();

frontend/src/pages/SynthesisDetail.tsx

@@ -2,6 +2,7 @@ import {
   type Component,
   createSignal,
   onMount,
+  onCleanup,
   Show,
   For,
 } from 'solid-js';
@@ -83,6 +84,12 @@ const SynthesisDetail: Component = () => {
   const [sendingEmail, setSendingEmail] = createSignal(false);
   const [emailSuccess, setEmailSuccess] = createSignal(false);
   const [emailError, setEmailError] = createSignal<string | null>(null);
+  const [emailTimer, setEmailTimer] = createSignal<ReturnType<typeof setTimeout> | undefined>();
+
+  onCleanup(() => {
+    const t = emailTimer();
+    if (t) clearTimeout(t);
+  });
 
   // Export state
   const [exportingMarkdown, setExportingMarkdown] = createSignal(false);
@@ -147,7 +154,7 @@ const SynthesisDetail: Component = () => {
     try {
       await synthesesApi.sendEmail(synth.id, email());
       setEmailSuccess(true);
-      setTimeout(() => setEmailSuccess(false), 5000);
+      setEmailTimer(setTimeout(() => setEmailSuccess(false), 5000));
     } catch (err) {
       if (isApiError(err)) {
         setEmailError(err.message);