feat(desktop): runtime-configurable backend server address

The desktop shell is one generic .deb/.AppImage distributed via
mca/public_releases, not built per-booth, but the backend origin was baked
in at build time (VITE_API_BASE, hardcoded to http://127.0.0.1:3000) — the
same installer could never point at a different appliance without a
rebuild.

Adds ConnectScreen (shown before Login in Tauri when no backend is saved),
backed by tauri-plugin-store persisting the operator-entered URL across
restarts. CSP's connect-src tightens to 'self' only — all backend traffic
already routes through tauri-plugin-http/websocket, which run Rust-side
and are outside connect-src's reach anyway — and the real access boundary
moves to capabilities/default.json's http:default scope, wildcarded so an
operator-chosen host is actually reachable. Adds a "Change server" control
in Setup (desktop-only) to repoint an already-configured install.

While tracing the desktop auth path for this: tauri-plugin-http's fetch()
runs through Rust's reqwest, which keeps its own cookie jar separate from
the webview, so document.cookie on tauri://localhost never sees the
parking_csrf cookie the server sets (open upstream bug,
tauri-apps/tauri#13045/#11518). This means the desktop app has likely been
silently sending no CSRF header on every mutation since the shell was
first built — pre-existing, independent of this change. Fixed by having
sessionView() (routes/auth.ts) also echo the CSRF value in the login/me
JSON body; the desktop client stashes it in memory and echoes that instead
of reading document.cookie. assertCsrf() itself is untouched.

Verified end-to-end against a real LAN-bound dev server: login returns a
csrfToken matching the cookie, a mutation using the body-sourced token in
X-CSRF-Token succeeds (200), and the same mutation without it still
correctly 403s.

pnpm-lock.yaml

@@ -117,6 +117,9 @@ importers:
       '@tauri-apps/plugin-process':
         specifier: ^2.3.1
         version: 2.3.1
+      '@tauri-apps/plugin-store':
+        specifier: ^2.4.0
+        version: 2.4.4
       '@tauri-apps/plugin-updater':
         specifier: ^2.10.1
         version: 2.10.1
@@ -1592,6 +1595,9 @@ packages:
   '@tauri-apps/plugin-process@2.3.1':
     resolution: {integrity: sha512-nCa4fGVaDL/B9ai03VyPOjfAHRHSBz5v6F/ObsB73r/dA3MHHhZtldaDMIc0V/pnUw9ehzr2iEG+XkSEyC0JJA==}
 
+  '@tauri-apps/plugin-store@2.4.4':
+    resolution: {integrity: sha512-oxSMaj/QpVfJcBMYX5aOQV94fWvga0MwQMfD6TLlbK2dh+ShPWAzefd8HWXhvOKjPRJdGVAkW7ZGO76JzzjaDA==}
+
   '@tauri-apps/plugin-updater@2.10.1':
     resolution: {integrity: sha512-NFYMg+tWOZPJdzE/PpFj2qfqwAWwNS3kXrb1tm1gnBJ9mYzZ4WDRrwy8udzWoAnfGCHLuePNLY1WVCNHnh3eRA==}
 
@@ -4112,6 +4118,10 @@ snapshots:
     dependencies:
       '@tauri-apps/api': 2.11.1
 
+  '@tauri-apps/plugin-store@2.4.4':
+    dependencies:
+      '@tauri-apps/api': 2.11.1
+
   '@tauri-apps/plugin-updater@2.10.1':
     dependencies:
       '@tauri-apps/api': 2.11.1