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).

hermes_cli/dashboard_auth/middleware.py

@@ -58,14 +58,73 @@ def _client_ip(request: Request) -> str:
     return request.client.host if request.client else ""
 
 
-def _unauth_response(path: str, *, reason: str) -> Response:
-    """API routes → 401 JSON; HTML routes → 302 → /login."""
+def _unauth_response(request: Request, *, reason: str) -> Response:
+    """API routes → 401 JSON with ``login_url``; HTML routes → 302 → /login.
+
+    The JSON envelope carries a ``login_url`` field with a ``next=`` query
+    string so the SPA's global 401 handler can drop the user back where
+    they were after re-auth. The contract is intentionally simple so any
+    fetch-wrapper can implement the redirect without parsing details:
+
+        if response.status === 401 && body.error in ("unauthenticated",
+                                                       "session_expired"):
+            window.location.assign(body.login_url);
+
+    HTML redirects also carry the ``next=`` query string so direct
+    navigation to ``/sessions`` (etc.) without a cookie comes back to
+    ``/sessions`` after login.
+    """
+    path = request.url.path
+    next_param = _safe_next_target(request)
+    login_url = f"/login?next={next_param}" if next_param else "/login"
+
     if path.startswith("/api/"):
+        # API routes never get redirects: the browser fetch() API would
+        # follow a 302 into the cross-origin OAuth dance opaquely. Return
+        # 401 with a structured envelope so the SPA can full-page-navigate
+        # to login_url.
+        error_code = (
+            "session_expired"
+            if reason == "invalid_or_expired_session"
+            else "unauthenticated"
+        )
         return JSONResponse(
-            {"detail": "Unauthorized", "reason": reason},
+            {
+                "error": error_code,
+                "detail": "Unauthorized",
+                "reason": reason,
+                "login_url": login_url,
+            },
             status_code=401,
         )
-    return RedirectResponse(url="/login", status_code=302)
+    return RedirectResponse(url=login_url, status_code=302)
+
+
+def _safe_next_target(request: Request) -> str:
+    """Build the URL-encoded ``next`` query value, or empty string.
+
+    Only same-origin relative paths are accepted; absolute URLs or
+    ``//evil.com`` open-redirect attempts are silently dropped. The empty
+    string return means the caller produces a bare ``/login`` URL — fine,
+    user lands at the dashboard root after re-auth.
+    """
+    path = request.url.path
+    # Reject anything that doesn't start with "/" or starts with "//"
+    # (protocol-relative URL — would open-redirect to an attacker host).
+    if not path or not path.startswith("/") or path.startswith("//"):
+        return ""
+    # Don't redirect back to the auth routes themselves — that loops.
+    if any(
+        path == p or path.startswith(p)
+        for p in ("/login", "/auth/", "/api/auth/")
+    ):
+        return ""
+    # Preserve query string if present (e.g. /sessions?page=2).
+    query = request.url.query
+    target = f"{path}?{query}" if query else path
+    # urlencode the whole thing as a single value.
+    from urllib.parse import quote
+    return quote(target, safe="")
 
 
 async def gated_auth_middleware(
@@ -86,7 +145,7 @@ async def gated_auth_middleware(
 
     at, _rt = read_session_cookies(request)
     if not at:
-        return _unauth_response(path, reason="no_cookie")
+        return _unauth_response(request, reason="no_cookie")
 
     # Try every registered provider's verify_session in turn. Providers
     # MUST return None for tokens they don't recognise (not raise). This
@@ -120,7 +179,14 @@ async def gated_auth_middleware(
             reason="no_provider_recognises",
             ip=_client_ip(request),
         )
-        return _unauth_response(path, reason="invalid_or_expired_session")
+        response = _unauth_response(request, reason="invalid_or_expired_session")
+        # Clear the dead cookie so the browser doesn't keep sending it.
+        # Contract v1: no refresh token to retry with, so the only correct
+        # next step is full re-auth via /login. Importing locally avoids a
+        # cycle with cookies → middleware at module load.
+        from hermes_cli.dashboard_auth.cookies import clear_session_cookies
+        clear_session_cookies(response)
+        return response
 
     request.state.session = session
     return await call_next(request)