feat: add PRIVATE_MODE site-wide auth gate (0.48.0)

When `PRIVATE_MODE=true`, every API path except a small allowlist
(`/health`, `/auth/{config,login,logout,register}`) requires a valid
session cookie or bearer token — anonymous reads are rejected with
401. Self-registration is force-disabled in private mode regardless
of `ALLOW_SELF_REGISTER`, so a locked-down instance flips with a
single switch (admins still mint accounts via `POST /admin/users`).

The backend gate is a tower middleware that reuses the existing
`CurrentUser` extractor, so the cookie + bearer paths cannot drift
from per-handler auth. `/auth/config` now exposes the flag plus the
effective `self_register_enabled` value so the frontend can render
the navbar correctly on the first paint.

On the frontend, a new universal root `+layout.ts` fetches the
config and redirects anonymous visitors to `/login?next=<path>`
before page-specific loads fire. The redirect is UX only — the
backend middleware is the source of truth, so crafted requests
still 401.

Defaults stay public (`PRIVATE_MODE=false`); existing deployments
need no env change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/api/auth.rs

@@ -42,18 +42,22 @@ pub fn routes() -> Router<AppState> {
         .route("/auth/tokens/:id", delete(delete_token))
 }
 
-/// Public, unauthenticated. Exposes anonymous-relevant auth policy
-/// (currently just whether self-registration is open) so the frontend
-/// can render its login / register affordances correctly without a
-/// probe request that would conflate "disabled" with "rate-limited".
+/// Public, unauthenticated. Exposes anonymous-relevant auth policy so
+/// the frontend can render its login / register affordances correctly
+/// without a probe request that would conflate "disabled" with
+/// "rate-limited". `self_register_enabled` is the *effective* value
+/// (`allow_self_register && !private_mode`), so a private-mode
+/// instance reports `false` even if the raw flag is on.
 #[derive(Debug, Serialize)]
 pub struct AuthConfigResponse {
     pub self_register_enabled: bool,
+    pub private_mode: bool,
 }
 
 async fn auth_config(State(state): State<AppState>) -> Json<AuthConfigResponse> {
     Json(AuthConfigResponse {
-        self_register_enabled: state.auth.allow_self_register,
+        self_register_enabled: state.auth.allow_self_register && !state.auth.private_mode,
+        private_mode: state.auth.private_mode,
     })
 }
 
@@ -103,7 +107,10 @@ async fn register(
     // disabled and enabled paths both consume a token, and disabled
     // returns 403 instead of running argon2.
     check_auth_rate_limit(&state, "register")?;
-    if !state.auth.allow_self_register {
+    // Private mode force-blocks self-registration regardless of
+    // ALLOW_SELF_REGISTER — operators of locked-down instances mint
+    // accounts via `POST /admin/users` instead.
+    if !state.auth.allow_self_register || state.auth.private_mode {
         return Err(AppError::Forbidden);
     }
     let username = input.username.trim();