fix(messages-ui): surface silent send failures, stop scroll/render churn (+cleanup)

- ChatThread send had no onError: a 404 (recipient gone) or 429 (rate-limited)
  send failed silently (api.req shows no modal for those "caller-handled" statuses).
  Added onError → toast for 404/429 (400/409/500 already raise the global dialog);
  draft is kept for retry. New trilingual messages.sendFailed key.
- ChatThread scroll-to-bottom effect keyed on `plain` too, so every 20s poll's
  decrypt pass yanked a scrolled-up reader back to the bottom. Key on items.length.
- ChatThread + Messages decrypt effects keyed on `items` (a fresh `?? []` array
  every render) → a render storm while loading. Key on q.dataUpdatedAt.
- chatDock.ts: use LS.chatDock(userId) instead of the bare "siftlode.chatDock.*"
  literal (matches the storage LS registry).

tsc green, re-review clean, localdev boots, Messages renders (locked-state) w/o console errors.

frontend/src/components/Messages.tsx

@@ -64,12 +64,15 @@ function ConversationList({
   const items = q.data?.items ?? [];
   const [previews, setPreviews] = useState<Record<number, string>>({});
 
+  // Decrypt conversation previews when fresh data arrives. Keyed on dataUpdatedAt (not `items`):
+  // the `?? []` fallback is a new array each render, which would otherwise spin this effect
+  // (and setPreviews) every render while the query is loading.
   useEffect(() => {
     if (!ready) return;
     let cancelled = false;
     (async () => {
       const out: Record<number, string> = {};
-      for (const c of items) {
+      for (const c of q.data?.items ?? []) {
         const m = c.last_message;
         if (m.kind === "user" && m.ciphertext && m.iv) {
           try {
@@ -85,7 +88,8 @@ function ConversationList({
     return () => {
       cancelled = true;
     };
-  }, [items, ready]);
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [q.dataUpdatedAt, ready]);
 
   function previewText(c: Conversation): string {
     const m = c.last_message;