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/utils/securityNudge.test.ts

@@ -0,0 +1,81 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import {
+  dismissNudge,
+  NUDGE_GRACE_MS,
+  NUDGE_SNOOZE_MS,
+  nudgeFor,
+  parseNudgeRecord,
+  SecurityState,
+  shouldShowNudge,
+} from './securityNudge';
+
+const healthy: SecurityState = {
+  crossSigning: true,
+  deviceVerified: true,
+  backupOnServer: true,
+  backupActive: true,
+};
+
+test('the #123 table: which nudge for which state', () => {
+  // E: healthy → nothing, ever.
+  assert.equal(nudgeFor(healthy), undefined);
+  // B: cross-signing, this device unverified, backup on server → verify first.
+  assert.equal(nudgeFor({ ...healthy, deviceVerified: false }), 'verify-device');
+  // C: cross-signing, unverified, no backup → still verify first.
+  assert.equal(
+    nudgeFor({ ...healthy, deviceVerified: false, backupOnServer: false }),
+    'verify-device',
+  );
+  // D: verified, no backup → #110's set-up nudge.
+  assert.equal(nudgeFor({ ...healthy, backupOnServer: false }), 'setup-backup');
+  // #110 state 2: backup exists, this device not using it.
+  assert.equal(nudgeFor({ ...healthy, backupActive: false }), 'connect-backup');
+  // A: never set up cross-signing, no backup → set up (the flow does both).
+  assert.equal(
+    nudgeFor({
+      crossSigning: false,
+      deviceVerified: false,
+      backupOnServer: false,
+      backupActive: false,
+    }),
+    'setup-backup',
+  );
+});
+
+test('nothing while crypto is still loading (F)', () => {
+  assert.equal(nudgeFor({ ...healthy, deviceVerified: undefined }), undefined);
+  assert.equal(nudgeFor({ ...healthy, backupOnServer: undefined }), undefined);
+  assert.equal(nudgeFor({ ...healthy, backupActive: undefined }), undefined);
+  // Unknown backup connection doesn't hide a known "no backup at all".
+  assert.equal(
+    nudgeFor({ ...healthy, backupOnServer: false, backupActive: undefined }),
+    'setup-backup',
+  );
+});
+
+test('timing: 24 h grace, 7-day snooze, stop after 3 dismissals', () => {
+  const t0 = 1_700_000_000_000;
+  let record = parseNudgeRecord(null, t0);
+  assert.equal(shouldShowNudge('verify-device', record, t0), false, 'first launch');
+  assert.equal(shouldShowNudge('verify-device', record, t0 + NUDGE_GRACE_MS - 1), false);
+  let now = t0 + NUDGE_GRACE_MS;
+  assert.equal(shouldShowNudge('verify-device', record, now), true, 'after 24 h');
+  for (let i = 1; i <= 3; i += 1) {
+    record = dismissNudge('verify-device', record, now);
+    assert.equal(shouldShowNudge('verify-device', record, now), false, `snoozed after #${i}`);
+    now += NUDGE_SNOOZE_MS;
+    assert.equal(shouldShowNudge('verify-device', record, now), i < 3, `after 7 days (#${i})`);
+  }
+  // Dismissing one nudge doesn't hide another.
+  assert.equal(shouldShowNudge('setup-backup', record, now), true);
+});
+
+test('stored record: round trip, and garbage starts a fresh grace period', () => {
+  const t0 = 1_700_000_000_000;
+  const r = dismissNudge('setup-backup', parseNudgeRecord(null, t0 - 5), t0);
+  assert.deepEqual(parseNudgeRecord(JSON.stringify(r), t0 + 1), r);
+  for (const bad of ['{', 'null', '"x"', '{"firstSeen":"yesterday"}', '{"firstSeen":1}']) {
+    assert.deepEqual(parseNudgeRecord(bad, t0), { firstSeen: t0, dismissals: {} }, bad);
+  }
+});