feat(auth): enforce session idle and absolute timeouts (#1387)

* feat(auth): enforce session idle and absolute timeouts

Hosted browser sessions now carry an HMAC-signed, HttpOnly cookie holding
session start, last activity and sign-in method, bound to the Supabase
session. Middleware enforces a 30 min idle and 12 h absolute limit
(reason-coded redirects to /login), a heartbeat route advances idle
activity from real user input, and a client controller warns 2 minutes
before expiry. BankID users are routed back to BankID on re-auth via a
short-lived method hint. API-key and MCP bearer surfaces are exempt;
self-hosted installs default off and can opt in via env vars.

Fixes #362

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(auth): derive session-timeout signing key via HKDF

The HMAC key is now HKDF-derived with a purpose-bound info string, so
the SUPABASE_SERVICE_ROLE_KEY fallback never uses the privileged
credential directly as a signing key. Addresses the security review
finding on PR #1387.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(auth): back signature bytes with a plain ArrayBuffer

crypto.subtle.verify requires a BufferSource; Uint8Array.from is typed
over ArrayBufferLike, which the Vercel TypeScript build rejects. Decode
base64url into a Uint8Array constructed over a fresh ArrayBuffer.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(auth): address session-timeout review findings

- signSessionTimeoutState returns null on signing failure instead of
  throwing, so a missing secret degrades the timeout feature in line
  with verifySessionTimeoutState rather than crashing authenticated
  requests; middleware and heartbeat skip the cookie write when null
- heartbeat initializes a fresh signed state for a missing or
  session-mismatched cookie, mirroring middleware, instead of
  returning SESSION_EXPIRED during normal initialization
- sessionStateMatchesUser treats an unresolved current session id as
  a mismatch for session-bound state so another session's cookie is
  never accepted on the userId fallback alone
- drop aria-live from the countdown DialogDescription so screen
  readers are not interrupted every second

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/auth/session-timeout-shared.ts

@@ -0,0 +1,28 @@
+export const SESSION_TIMEOUT_COOKIE = 'gnubok-session-timeout'
+export const SESSION_AUTH_METHOD_HINT_COOKIE = 'gnubok-auth-method'
+export const SESSION_TIMEOUT_CHANNEL = 'gnubok-session-timeout'
+
+export type SessionAuthMethod = 'password' | 'bankid'
+export type SessionTimeoutReason = 'idle' | 'absolute'
+
+export interface SessionTimeoutClientState {
+  enabled: boolean
+  idleTimeoutMs: number
+  absoluteTimeoutMs: number
+  warningMs: number
+  serverNow: number
+  startedAt: number
+  lastActivityAt: number
+  method: SessionAuthMethod
+}
+
+export function isSessionAuthMethod(value: unknown): value is SessionAuthMethod {
+  return value === 'password' || value === 'bankid'
+}
+
+export function setSessionAuthMethodHint(method: SessionAuthMethod): void {
+  if (typeof document === 'undefined') return
+
+  const secure = window.location.protocol === 'https:' ? '; Secure' : ''
+  document.cookie = `${SESSION_AUTH_METHOD_HINT_COOKIE}=${method}; Path=/; Max-Age=300; SameSite=Lax${secure}`
+}