feat: device-security nudge — verify this device / key backup (#110, #123)

One "security" strip, in the same top slot and style as the status banner
(#124), for this device, in priority order:
- "Verify this device" — cross-signing exists but this device isn't
  verified (can't unlock backed-up history, untrusted to others). First,
  because verifying with the recovery key also connects the backup.
- "Connect this device to your key backup" — a backup exists, this device
  isn't using it.
- "Set up key backup" — no backup at all (includes accounts without
  cross-signing; the setup flow does both).
The button opens Settings → Devices (existing flows); "Not now" snoozes.

Rules: nothing in a device's first 24 h; "Not now" snoozes that nudge 7
days; 3 dismissals stop it; never on a healthy device; waits until sync
has settled (never under "Connecting…"); outage/maintenance/connection
strips win. Per-device record in localStorage, wiped on logout. Mounted
inside the Matrix client context and an error boundary (an early version
outside the context crashed the app on load — caught before pushing).

Also: the status strip wraps its text on phones instead of truncating it
when there's no Details button (benefits #124's strips too).

Design approved on #123 (real-client screenshots there). Tests: 4 unit
(the #123 state table, loading, timing, stored record); e2e: nothing during
the grace period, "Set up key backup" → Settings → Devices, "Not now"
holds across a reload. Unit 1319, Playwright 29 passed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/state/plaintextCaches.ts

@@ -8,6 +8,7 @@ import { clearNavToActivePathStore } from './navToActivePath';
 import { DRAFT_MSG_KEY_PREFIX } from '../utils/draft';
 import { clearCallSession } from '../utils/callRejoin';
 import { clearOutbox } from '../utils/outbox';
+import { NUDGE_KEY_PREFIX } from '../utils/securityNudge';
 
 /**
  * [Gitea #41] Wipe every persisted composer draft (`draft-msg-<roomId>`). Drafts
@@ -77,7 +78,11 @@ const clearStatusMessage = (): void => {
     return;
   }
   keys.forEach((key) => {
-    if (key.startsWith('lotus-status-msg-') || key.startsWith('lotus-status-expiry-')) {
+    if (
+      key.startsWith('lotus-status-msg-') ||
+      key.startsWith('lotus-status-expiry-') ||
+      key.startsWith(NUDGE_KEY_PREFIX)
+    ) {
       try {
         localStorage.removeItem(key);
       } catch {