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>

Dockerfile

@@ -38,6 +38,9 @@ ENV NEXT_PUBLIC_APP_URL=__NEXT_PUBLIC_APP_URL__
 ENV NEXT_PUBLIC_VAPID_PUBLIC_KEY=__NEXT_PUBLIC_VAPID_PUBLIC_KEY__
 ENV NEXT_PUBLIC_SELF_HOSTED=__NEXT_PUBLIC_SELF_HOSTED__
 ENV NEXT_PUBLIC_REQUIRE_MFA=__NEXT_PUBLIC_REQUIRE_MFA__
+ENV NEXT_PUBLIC_SESSION_IDLE_TIMEOUT_MS=__NEXT_PUBLIC_SESSION_IDLE_TIMEOUT_MS__
+ENV NEXT_PUBLIC_SESSION_ABSOLUTE_TIMEOUT_MS=__NEXT_PUBLIC_SESSION_ABSOLUTE_TIMEOUT_MS__
+ENV NEXT_PUBLIC_SESSION_WARNING_MS=__NEXT_PUBLIC_SESSION_WARNING_MS__
 # Keep the branding placeholder intact through prebuild's inject script so
 # docker-entrypoint.sh can substitute the runtime value into public/sw.js.
 ENV NEXT_PUBLIC_BRANDING_APP_NAME=__NEXT_PUBLIC_BRANDING_APP_NAME__