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/ChatThread.tsx

@@ -2,9 +2,10 @@ import { useEffect, useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQueryClient } from "@tanstack/react-query";
 import { Send } from "lucide-react";
-import { api, type Message, type MessageUser } from "../lib/api";
+import { api, HttpError, type Message, type MessageUser } from "../lib/api";
 import { useLiveQuery } from "../lib/useLiveQuery";
 import { relativeTime } from "../lib/format";
+import { notify } from "../lib/notifications";
 import { partnerPub, useKeyState } from "../lib/messaging";
 import * as e2ee from "../lib/e2ee";
 import KeyGate from "./KeyGate";
@@ -39,12 +40,15 @@ export default function ChatThread({
   );
   const items = q.data?.items ?? [];
 
+  // Decrypt the thread when fresh server data arrives. Keyed on dataUpdatedAt (not `items`): the
+  // `?? []` fallback is a new array reference every render, which would otherwise re-run this
+  // effect (and setPlain a new object) on every render while data is loading — a render storm.
   useEffect(() => {
     if (isSystem || !ready) return;
     let cancelled = false;
     (async () => {
       const out: Record<number, string> = {};
-      for (const m of items) {
+      for (const m of q.data?.items ?? []) {
         if (m.ciphertext && m.iv) {
           try {
             const pub = await partnerPub(partnerId);
@@ -59,7 +63,8 @@ export default function ChatThread({
     return () => {
       cancelled = true;
     };
-  }, [items, ready, isSystem, partnerId, t]);
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [q.dataUpdatedAt, ready, isSystem, partnerId, t]);
 
   // Opening / refreshing the thread marks incoming messages read, so clear the badges.
   useEffect(() => {
@@ -69,9 +74,12 @@ export default function ChatThread({
     }
   }, [q.dataUpdatedAt, qc]);
 
+  // Scroll to the newest message when the count changes (open / new message) — NOT on `plain`,
+  // which gets a fresh object on every poll's decrypt pass and would yank a scrolled-up reader
+  // back to the bottom even when nothing new arrived.
   useEffect(() => {
     bottomRef.current?.scrollIntoView({ block: "end" });
-  }, [items.length, plain]);
+  }, [items.length]);
 
   const send = useMutation({
     mutationFn: async (text: string) => {
@@ -84,6 +92,14 @@ export default function ChatThread({
       qc.invalidateQueries({ queryKey: ["thread", partnerId] });
       qc.invalidateQueries({ queryKey: ["conversations"] });
     },
+    onError: (e) => {
+      // The draft is kept (only cleared on success) so the user can retry. 400/409/422/500 already
+      // raised the global error dialog; 404 (recipient gone) and 429 (rate-limited) are "caller-
+      // handled" in api.req (no modal), so surface those here or the send fails silently.
+      if (e instanceof HttpError && (e.status === 404 || e.status === 429)) {
+        notify({ level: "error", message: e.detail || t("messages.sendFailed") });
+      }
+    },
   });
 
   const submit = () => {