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>

frontend/src/routes/+layout.svelte

@@ -14,15 +14,23 @@
     import Shield from '@lucide/svelte/icons/shield';
     import '$lib/styles/tokens.css';
 
-    let { children } = $props();
+    let { children, data } = $props();
     let loggingOut = $state(false);
     let headerEl: HTMLElement | undefined = $state();
 
+    // Seed authConfig from the universal layout load. $effect keeps
+    // the store in sync if `data` is replaced by a subsequent layout
+    // load (client-side nav). The first run also covers initial
+    // hydration so the navbar's register link reflects the real
+    // server flag without a separate fetch.
+    $effect(() => {
+        authConfig.seed(data.authConfig);
+    });
+
     onMount(() => {
         theme.init();
         preferences.init();
         if (!session.loaded) session.refresh();
-        if (!authConfig.loaded) authConfig.load();
 
         // Publish the header's measured height as a CSS custom
         // property so sticky descendants (e.g. the reader nav) can