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-tauri/src/preferences.rs

@@ -0,0 +1,31 @@
+use serde::{Deserialize, Serialize};
+use tauri::{AppHandle, Manager};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Reader {
+    pub host: String,
+    pub port: u16,
+}
+
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+struct Preferences {
+    #[serde(default)]
+    reader: Vec<Reader>,
+}
+
+pub fn load(app: &AppHandle) -> Option<Reader> {
+    let path = app.path().app_data_dir().ok()?.join("preferences.toml");
+    let contents = std::fs::read_to_string(path).ok()?;
+    let preferences: Preferences = toml::from_str(&contents).ok()?;
+    preferences.reader.into_iter().next()
+}
+
+pub fn save(app: &AppHandle, reader: Reader) -> Result<(), String> {
+    let dir = app.path().app_data_dir().map_err(|err| err.to_string())?;
+    std::fs::create_dir_all(&dir).map_err(|err| err.to_string())?;
+    let preferences = Preferences {
+        reader: vec![reader],
+    };
+    let contents = toml::to_string_pretty(&preferences).map_err(|err| err.to_string())?;
+    std::fs::write(dir.join("preferences.toml"), contents).map_err(|err| err.to_string())
+}