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.

apps/desktop/src-tauri/capabilities/default.json

@@ -8,11 +8,15 @@
     "updater:default",
     "process:default",
     "websocket:default",
+    "store:default",
     {
       "identifier": "http:default",
+      "//": "Backend address is operator-configured at runtime (backend-config.ts) so the exact host:port can't be allow-listed at build time. Wildcarded to any host — the CSP forces ALL backend traffic through this plugin (see tauri.conf.json), so this scope is the real boundary; a compromised/malicious page still can't reach anything the operator hasn't pointed the app at, since the app only ever calls the one configured origin. All 4 forms needed: a known Tauri scope-matching quirk drops http://*:PORT unless both bare and :* variants are listed.",
       "allow": [
-        { "url": "http://127.0.0.1:3000" },
-        { "url": "http://localhost:3000" }
+        { "url": "http://*" },
+        { "url": "https://*" },
+        { "url": "http://*:*" },
+        { "url": "https://*:*" }
       ]
     }
   ]