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/src/lib/locales/en.json

@@ -0,0 +1,25 @@
+{
+  "loading": "Loading...",
+  "onboarding": {
+    "title": "Welcome!",
+    "subtitle": "Connect Oxibadge to your time clock by entering its IP address and port.",
+    "host_label": "IP address",
+    "port_label": "Port",
+    "hint": "You can find the address in the time clock's network settings.",
+    "button": {
+      "idle": "Test connection",
+      "testing": "Connecting...",
+      "retry": "Retry"
+    },
+    "error_title": "Could not reach the time clock",
+    "errors": {
+      "connection-refused": "Connection refused",
+      "timed-out": "Connection timed out",
+      "dns-failure": "Invalid or unreachable address",
+      "unknown": "Connection error"
+    }
+  },
+  "done": {
+    "message": "Nothing to do here."
+  }
+}