feat: AccountSecuritySettingsPage + active-users list + toast + login banner

Eighth commit in the session-expiration-policy series. Surfaces all
the owner controls and user-facing expiry UX that the prior commits
plumbed through, designed end-to-end via /plan-design-review (initial
4/10 -> final 9/10; 7 decisions locked in the plan).

Backend additions:
- accounts/me/security GET response gains active_users: list of
  {user_id, name, email, last_login_at} for users in this account
  with at least one un-revoked refresh token. Joined query on
  refresh_tokens + users, distinct, ordered by last_login desc.
  Drives the Active Sessions section.

Frontend additions:
- api/accountSecurity.ts: typed client for GET/PATCH/revoke-sessions.
- hooks/useAuthSessionExpiry.ts: reads idle/absolute expiry from the
  auth store, returns warning ('none'|'soon'|'now') + reason
  ('idle'|'absolute') so consumers can pick the right UX for the
  closer window. Re-evaluates every 30s.
- components/common/SessionExpiryToast.tsx: top-of-app notice that
  fires at T-5min. Idle case: warning-amber tone, [Stay signed in]
  button hits authApi.refresh() and updates the store on success.
  Absolute case: info-cyan tone, [Sign in now] link to /login (no
  recoverable action). Dismissable, doesn't re-fire after dismissal.
- components/account/RevokeSessionsModal.tsx: confirmation modal for
  the two bulk-revoke scopes. Title, body, and confirm-label vary by
  scope; danger-styled confirm button.
- pages/account/AccountSecuritySettingsPage.tsx: the main page.
  Header (Shield icon), intro, Policy card with Strict/Standard/Custom
  radios + always-visible-disabled Custom inputs (idle/absolute
  minutes) with inline validation, Save button + emerald success ping,
  info note about 'applies at next login'. Active sessions card with
  count-aware copy, list of {name, email, last-login-ago} rows
  (caller tagged '(you)'), two buttons — 'except me' hidden when
  count=1, 'sign me out and everyone else' uses danger-tinted styling.
- pages/AccountSettingsPage.tsx: 'Session security' row added to the
  owner-only settings list.
- router.tsx: /account/security route, owner-gated via ProtectedRoute.
- pages/LoginPage.tsx: cyan info-tone banner above form when
  ?reason=session_expired is in the URL.
- components/layout/AppLayout.tsx: mounts <SessionExpiryToast />.

Scope=all bulk-revoke UX (the most jarring moment): on success,
toast.success(N sessions), 1.5s delay, then clear localStorage +
useAuthStore.logout() + window.location='/login' (no banner — the
owner just did this).

Backend tests: existing 22/22 still green plus the GET test now
asserts active_users is present + non-empty after login. Frontend:
tsc clean, authStore test 2/2.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/schemas/account_security.py

@@ -2,11 +2,28 @@
 
 See docs/plans/2026-05-13-session-expiration-policy.md §4.7 and §4.11.
 """
+from datetime import datetime
 from typing import Literal, Optional
+from uuid import UUID
 
 from pydantic import BaseModel, Field
 
 
+class ActiveUser(BaseModel):
+    """One row in the active-users list on GET /accounts/me/security.
+
+    Rendered as 'name (email) · logged in 2d ago' on the Account Security
+    page. `last_login_at` reflects the last successful sign-in, not the last
+    refresh-token use — that requires the deferred refresh_tokens.last_used_at
+    follow-up (see plan §9).
+    """
+
+    user_id: UUID
+    name: str
+    email: str
+    last_login_at: Optional[datetime] = None
+
+
 class SessionPolicyResponse(BaseModel):
     """GET /accounts/me/security — the policy in effect for this account.
 
@@ -32,6 +49,10 @@ class SessionPolicyResponse(BaseModel):
     absolute_minutes_min: int
     absolute_minutes_max: int
 
+    # Active sessions in this account — users with at least one un-revoked
+    # refresh token. Drives the Active Sessions section in the UI.
+    active_users: list[ActiveUser] = Field(default_factory=list)
+
 
 class SessionPolicyUpdateRequest(BaseModel):
     """PATCH /accounts/me/security — set or clear the per-account override.