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/lib/chatDock.ts

@@ -3,6 +3,7 @@
 // when a message arrives) and stays open across page navigation. The dock state (which chats
 // are open + their minimised state) is persisted per user so it survives a reload.
 import { useSyncExternalStore } from "react";
+import { LS } from "./storage";
 
 export interface DockChat {
   partnerId: number;
@@ -20,7 +21,7 @@ let userId: number | null = null;
 const subs = new Set<() => void>();
 
 function storageKey(): string | null {
-  return userId != null ? `siftlode.chatDock.${userId}` : null;
+  return userId != null ? LS.chatDock(userId) : null;
 }
 
 function persist(): void {