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/routes/+page.svelte

@@ -1,33 +1,26 @@
-<main>
-  <img src="/logo.svg" alt="Oxibadge logo" class="logo" />
-  <h1>Hello, world!</h1>
-</main>
+<script lang="ts">
+  import { invoke } from '@tauri-apps/api/core';
+  import { onMount } from 'svelte';
+  import { _ } from 'svelte-i18n';
+  import Screen from '$lib/components/Screen.svelte';
+  import Onboarding from '$lib/components/Onboarding.svelte';
 
-<style>
-  :global(body) {
-    margin: 0;
-    color: #f6f6f6;
-    background-color: #2f2f2f;
-    font-family: Inter, Avenir, Helvetica, Arial, sans-serif;
-  }
+  type Step = 'loading' | 'setup' | 'done';
 
-  main {
-    min-height: 100vh;
-    display: flex;
-    flex-direction: column;
-    align-items: center;
-    justify-content: center;
-    gap: 1.5rem;
-  }
+  let step = $state<Step>('loading');
 
-  .logo {
-    width: 8rem;
-    height: 8rem;
-  }
+  onMount(async () => {
+    const preferences = await invoke<{ host: string; port: number } | null>('load_preferences');
+    step = preferences ? 'done' : 'setup';
+  });
+</script>
 
-  h1 {
-    margin: 0;
-    font-size: 2rem;
-    font-weight: 500;
-  }
-</style>
+<Screen>
+  {#if step === 'loading'}
+    <p>{$_('loading')}</p>
+  {:else if step === 'setup'}
+    <Onboarding onconnected={() => (step = 'done')} />
+  {:else}
+    <p>{$_('done.message')}</p>
+  {/if}
+</Screen>