feat(security): nudge to sign out sessions unused for 3 months (#113)

Sessions of this account not seen for 90+ days (never the current one; a
session with no last-seen time is left alone) get a nudge in the existing
security strip, after the device-security ones and with the same rules
("Not now" snoozes 7 days, gone after 3 dismissals; a snoozed security
nudge no longer hides it): "You have 3 sessions you haven't used in 3
months. Signing them out keeps your account safe if an old device is
lost." [Review]

Review opens Settings -> Devices with those sessions ticked for sign-out
(one confirm, the usual re-auth incl. SSO); they are listed first and
marked "Inactive for 3+ months". OIDC accounts sign out on the
provider's dashboard, so nothing is ticked there.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/utils/securityNudge.ts

@@ -9,12 +9,15 @@
  * - setup-backup: no key backup at all (includes accounts that never set up
  *   cross-signing: the setup flow does both).
  *
+ * - stale-sessions (#113, lowest priority): sessions of this account not used
+ *   for 90+ days; Review opens Settings → Devices with them ticked to sign out.
+ *
  * Anything not yet known (crypto still loading) → no nudge. Timing: not in
  * the first 24 h on a device (onboarding), "Not now" snoozes 7 days, three
  * dismissals of a nudge stop it for good; a healthy device never sees one.
  */
 
-export type NudgeKind = 'verify-device' | 'connect-backup' | 'setup-backup';
+export type NudgeKind = 'verify-device' | 'connect-backup' | 'setup-backup' | 'stale-sessions';
 
 export type SecurityState = {
   /** m.cross_signing.master account data present. */
@@ -38,6 +41,51 @@ export const nudgeFor = (s: SecurityState): NudgeKind | undefined => {
   return s.backupActive ? undefined : 'connect-backup';
 };
 
+/** [Gitea #113] A session counts as stale after this long unused. */
+export const STALE_SESSION_MS = 90 * 24 * 60 * 60 * 1000;
+
+type SessionLike = { device_id: string; last_seen_ts?: number | null };
+
+/**
+ * Pure: other sessions of this account not seen for STALE_SESSION_MS. Never
+ * the current session; a session with no last-seen time is left alone (we
+ * can't tell it's unused).
+ */
+export const staleSessions = <T extends SessionLike>(
+  devices: T[],
+  currentDeviceId: string | undefined,
+  now: number,
+): T[] =>
+  devices.filter(
+    (d) =>
+      d.device_id !== currentDeviceId &&
+      typeof d.last_seen_ts === 'number' &&
+      now - d.last_seen_ts >= STALE_SESSION_MS,
+  );
+
+/** "1 session" / "3 sessions you haven't used in 3 months". */
+export const staleSessionsText = (count: number): string =>
+  `You have ${count === 1 ? 'a session' : `${count} sessions`} you haven't used in 3 months. Signing ${
+    count === 1 ? 'it' : 'them'
+  } out keeps your account safe if an old device is lost.`;
+
+/**
+ * The nudge to show: the device-security one first, then stale sessions, each
+ * only if its own snooze allows (so a snoozed security nudge doesn't hide the
+ * stale-sessions one, and vice versa).
+ */
+export const pickNudge = (
+  security: NudgeKind | undefined,
+  staleCount: number,
+  record: NudgeRecord,
+  now: number,
+): NudgeKind | undefined => {
+  const candidates: NudgeKind[] = [];
+  if (security) candidates.push(security);
+  if (staleCount > 0) candidates.push('stale-sessions');
+  return candidates.find((k) => shouldShowNudge(k, record, now));
+};
+
 export const NUDGE_GRACE_MS = 24 * 60 * 60 * 1000;
 export const NUDGE_SNOOZE_MS = 7 * 24 * 60 * 60 * 1000;
 export const NUDGE_MAX_DISMISSALS = 3;
@@ -87,6 +135,11 @@ export const NUDGE_COPY: Record<NudgeKind, { text: string; action: string }> = {
     text: "This device isn't using your key backup yet. Connect it so your encrypted messages stay readable if you lose it.",
     action: 'Connect',
   },
+  'stale-sessions': {
+    // The text depends on the count: see staleSessionsText.
+    text: '',
+    action: 'Review',
+  },
   'setup-backup': {
     text: "Your encryption keys aren't backed up. Set up key backup so you don't lose your encrypted messages if you log out or lose this device.",
     action: 'Set up',