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/components/Screen.svelte

@@ -0,0 +1,20 @@
+<script lang="ts">
+  import type { Snippet } from 'svelte';
+
+  let { children }: { children: Snippet } = $props();
+</script>
+
+<main class="screen">
+  {@render children()}
+</main>
+
+<style>
+  .screen {
+    min-height: 100vh;
+    box-sizing: border-box;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    padding: 48px 24px;
+  }
+</style>