feat(oxibadge): add onboarding flow with i18n and reader preferences

Adds the first real screen: a setup form for the device's host/port,
tested against the real device (3 retries) and persisted to
preferences.toml before showing a placeholder "done" screen.

- Rust: test_connection/save_preferences/load_preferences commands;
  preferences stored as [[reader]] in TOML so more readers can be
  added later without a format change.
- Svelte: split the monolithic page into reusable components
  (Screen, Column, Field, Button, ErrorBanner, Hint) under
  src/lib/components, sharing design tokens from app.css.
- Automatic it/en i18n via svelte-i18n, locale detected from
  navigator.language.

Assisted-by: Claude Sonnet 5 <noreply@anthropic.com>

oxibadge/package.json

@@ -13,6 +13,13 @@
     "tauri": "tauri"
   },
   "license": "AGPL-3.0-or-later",
+  "dependencies": {
+    "@fontsource/bricolage-grotesque": "^5.3.0",
+    "@fontsource/geist": "^5.3.0",
+    "@fontsource/geist-mono": "^5.3.0",
+    "@tauri-apps/api": "^2",
+    "svelte-i18n": "^4.0.1"
+  },
   "devDependencies": {
     "@sveltejs/adapter-static": "^3.0.10",
     "@sveltejs/kit": "^2.65.1",