feat(dashboard-auth): Phase 6 — 401 re-auth envelope + next= propagation

Contract V1 of nous-account-service PR #180 ships no refresh tokens, so
the original Phase 6 silent-refresh design is replaced with a thinner
'401 → redirect to /login' UX. The dashboard's gated middleware now
emits a structured envelope on any auth failure; the SPA's fetch
wrapper sees it and full-page-navigates the user through re-auth.

hermes_cli/dashboard_auth/cookies.py:
  set_session_cookies(refresh_token='') SKIPS writing the
  hermes_session_rt cookie. Forward-compat: a non-empty refresh_token
  still emits the cookie unchanged, so a future Portal contract that
  starts issuing RTs flips the persistence on with no other change.
  clear_session_cookies still emits a Max-Age=0 deletion for the RT
  cookie so stale cookies from earlier deployments get flushed on
  logout / session expiry. Deprecation marker + rationale in
  module docstring per the user's docstring-only deprecation pattern.

hermes_cli/dashboard_auth/middleware.py:
  _unauth_response now builds a structured JSON envelope for API 401s:
    { error: 'session_expired' | 'unauthenticated',
      detail: 'Unauthorized',
      reason: <internal>,
      login_url: '/login?next=<safe-path>' }
  HTML redirects also carry next= so a user landing on /sessions
  without a cookie bounces back to /sessions after re-auth.
  _safe_next_target validates same-origin: drops protocol-relative
  paths (//evil.com), absolute URLs, and any /login or /auth/* loop.
  Dead cookies are cleared on the 401 path so the browser stops
  replaying invalid tokens.

hermes_cli/dashboard_auth/routes.py:
  /auth/callback accepts next= query param and validates via
  _validate_post_login_target (same rules as the gate's
  _safe_next_target — defence-in-depth because next= survived a full
  IDP round trip and attacker-controlled state can re-enter via the
  callback URL). Open-redirect attempts land at '/' instead.

web/src/lib/api.ts:
  fetchJSON parses the 401 envelope and full-page-navigates to
  body.login_url ONLY on the known session-expiry error codes.
  Domain-level 401s (e.g. permission errors) bubble up as regular
  errors. credentials: 'include' added so cookie auth works for all
  fetches routed through this wrapper. sessionStorage.lastLocation is
  preserved for future use by AuthWidget / hermes_status.

Test files marked with pytest.mark.xdist_group so the four files that
mutate web_server.app.state.auth_required serialize onto the same xdist
worker — eliminates 'works locally, fails in CI' app-state bleed.

20 new tests in test_dashboard_auth_401_reauth.py:
  - set_session_cookies(refresh_token='') skips RT cookie
  - clear_session_cookies still emits RT deletion
  - 401 envelope shape (unauthenticated vs session_expired)
  - dead cookie cleared on invalid-token 401
  - login_url carries next= for deep paths
  - login loop avoided when path is /login/auth/api-auth
  - protocol-relative URL rejected
  - _safe_next_target unit tests (accept same-origin, reject loops/abs)
  - /auth/callback respects safe next= but rejects open redirects

2 pre-existing tests updated to accept the new /login?next=%2F shape.

Full dashboard-auth suite: 168 passed, 1 skipped (Phase 0 pre-existing).

tests/hermes_cli/test_dashboard_auth_middleware.py

@@ -15,6 +15,13 @@ without any external IDP.  Exercises:
 from __future__ import annotations
 
 import pytest
+
+# Phase 5 / Phase 6: these tests mutate ``web_server.app.state.auth_required``
+# at module level. Run them in the same xdist worker so they don't race
+# against each other (and against any other file that also touches
+# ``app.state``) — the marker name is shared across all dashboard-auth test
+# files that gate the app.
+pytestmark = pytest.mark.xdist_group("dashboard_auth_app_state")
 from fastapi.testclient import TestClient
 
 from hermes_cli import web_server
@@ -58,7 +65,8 @@ def test_gated_status_now_requires_auth(gated_app):
 def test_gated_html_redirects_to_login(gated_app):
     r = gated_app.get("/", follow_redirects=False)
     assert r.status_code == 302
-    assert r.headers["location"] == "/login"
+    # Phase 6: gate carries a ``next=`` so post-login bounces back to /.
+    assert r.headers["location"] in ("/login", "/login?next=%2F")
 
 
 def test_gated_auth_providers_is_public(gated_app):
@@ -177,7 +185,8 @@ def test_invalid_cookie_redirects_on_html(gated_app):
     gated_app.cookies.set(SESSION_AT_COOKIE, "garbage")
     r = gated_app.get("/", follow_redirects=False)
     assert r.status_code == 302
-    assert r.headers["location"] == "/login"
+    # Phase 6: gate carries a ``next=`` so post-login bounces back to /.
+    assert r.headers["location"] in ("/login", "/login?next=%2F")
 
 
 def test_logout_clears_cookies_and_redirects_to_login(gated_app):