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/Button.svelte

@@ -0,0 +1,93 @@
+<script lang="ts">
+  let {
+    label,
+    type = 'button',
+    faded = false,
+    busy = false,
+    disabled = false,
+    onclick
+  }: {
+    label: string;
+    type?: 'button' | 'submit';
+    faded?: boolean;
+    busy?: boolean;
+    disabled?: boolean;
+    onclick?: () => void;
+  } = $props();
+</script>
+
+<button {type} class="btn" class:faded class:busy {disabled} {onclick}>
+  <span>{label}</span>
+  {#if busy}
+    <span class="spinner"></span>
+  {:else}
+    <svg width="18" height="18" viewBox="0 0 24 24" stroke-width="2" aria-hidden="true"
+      ><path d="M5 12h14M13 6l6 6-6 6" /></svg
+    >
+  {/if}
+</button>
+
+<style>
+  .btn {
+    all: unset;
+    box-sizing: border-box;
+    height: var(--ox-control-h);
+    width: 100%;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    gap: 10px;
+    border-radius: var(--ox-radius);
+    background: var(--ox-accent);
+    color: var(--ox-on-accent);
+    font-family: var(--ox-font-body);
+    font-size: 15px;
+    font-weight: 600;
+    cursor: pointer;
+    transition:
+      filter 0.15s,
+      transform 0.1s,
+      opacity 0.2s;
+  }
+
+  .btn:not(:disabled):hover {
+    filter: brightness(1.08);
+  }
+
+  .btn:not(:disabled):active {
+    transform: translateY(1px);
+  }
+
+  .btn:focus-visible {
+    outline: 2px solid #ffffff;
+    outline-offset: 3px;
+  }
+
+  .btn:disabled {
+    cursor: not-allowed;
+  }
+
+  .btn.faded {
+    opacity: 0.35;
+  }
+
+  .btn.busy {
+    opacity: 0.7;
+    cursor: progress;
+  }
+
+  .spinner {
+    width: 16px;
+    height: 16px;
+    border: 2px solid rgba(26, 7, 0, 0.35);
+    border-top-color: var(--ox-on-accent);
+    border-radius: 50%;
+    animation: spin 0.7s linear infinite;
+  }
+
+  @keyframes spin {
+    to {
+      transform: rotate(360deg);
+    }
+  }
+</style>