feat: v0.3.0 - fix provider data loss, oauth corruption & switch UX

Bug fixes:
- Fix config.toml wiped to single provider on switch (now writes all providers)
- Fix managed:kimi-code oauth key corruption (oauth/kimi-code was rewritten
  to oauth/managed-kimi-code, breaking the official subscription)
- Fix default_model not switching due to React 18 deferred state update
  (updateConfig now sets configRef synchronously before setConfig)
- Strip KimiSwitch-private default_model from config.toml provider output

Architecture:
- config.toml is now the authoritative source for Kimi Code provider data
- SQLite stores only metadata (notes, official URLs, remembered models)
- Load reconciles config.toml with SQLite for external /provider changes
- Activate is a no-op for KimiCode (save already writes config.toml)

Features:
- Selected provider moves to top of list on switch
- Add models-dev context size lookup
- Add tauri-plugin-single-instance

Tests:
- 3 new regression tests: oauth key round-trip, all-providers write,
  private default_model strip

src-tauri/src/lib.rs

@@ -11,6 +11,16 @@ use tauri::Manager;
 
 pub fn run() {
     tauri::Builder::default()
+        // Single instance: must be registered before other plugins. When a
+        // second process is launched, this callback runs in the existing
+        // instance and simply brings its window back (including from tray).
+        .plugin(tauri_plugin_single_instance::init(|app, _args, _cwd| {
+            if let Some(window) = app.get_webview_window("main") {
+                let _ = window.unminimize();
+                let _ = window.show();
+                let _ = window.set_focus();
+            }
+        }))
         .plugin(tauri_plugin_opener::init())
         .setup(|app| {
             println!("[Tauri] Setup started");