feat(web): login-gated BFF auth — sealed session cookie + server-side token injection

Single-user, LAN-reachable-but-gated. The web server is a backend-for-frontend:

- Login: POST /_auth/login {password} checks PUNKTFUNK_UI_PASSWORD (constant-time) and
  sets a SEALED session cookie (h3 useSession / AES-GCM). server/middleware/auth.ts gates
  every request — pages 302 → /login, /api → 401 — and FAILS CLOSED (503) when
  PUNKTFUNK_UI_PASSWORD is unset, so a misconfigured LAN-exposed server admits no one.
- The management API stays loopback-only + token (never LAN-exposed). The proxy
  (server/routes/api/[...].ts) injects PUNKTFUNK_MGMT_TOKEN server-side and drops the
  browser's cookie before forwarding — the token never reaches the browser, which only
  holds the session cookie.

Nitro doesn't auto-scan a server/ dir, so the Nitro plugin gets an explicit scanDirs to
pick up middleware + routes. Client: removed the localStorage token (server injects it);
the fetcher bounces to /login on 401; new /login page (bare, no shell); Settings drops the
token field and gains a Sign-out button; en/de strings.

Validated live end to end: unauth /→302, /api→401; wrong pw→401; right pw→200+cookie;
authed /api/v1/status→200 (proxied, mgmt token injected — the host required it); logout→
session cleared→401. tsc + build green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/messages/de.json

@@ -54,5 +54,12 @@
   "common_retry": "Erneut versuchen",
   "common_yes": "Ja",
   "common_cancel": "Abbrechen",
-  "common_unauthorized": "Nicht autorisiert — gültiges API-Token in den Einstellungen setzen."
+  "common_unauthorized": "Sitzung abgelaufen — Weiterleitung zur Anmeldung…",
+  "login_title": "Anmelden",
+  "login_subtitle": "Gib das Verwaltungspasswort ein, um fortzufahren.",
+  "login_password": "Passwort",
+  "login_submit": "Anmelden",
+  "login_error": "Falsches Passwort.",
+  "login_signing_in": "Anmeldung läuft…",
+  "action_logout": "Abmelden"
 }