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/i18n/locales/en/messages.json

@@ -17,6 +17,7 @@
   "encrypted": "Encrypted message",
   "encryptedHint": "End-to-end encrypted",
   "cantDecrypt": "Can't decrypt",
+  "sendFailed": "Couldn't send your message. Try again.",
   "systemReadOnly": "This is an automated message.",
   "setupTitle": "Set up secure messaging",
   "setupBody": "Messages are end-to-end encrypted. Choose a passphrase to protect your key — it never leaves your device, so no one (not even an admin) can read your conversations.",