feat(settings): add option to skip Claude Code first-run confirmation

Add a new setting to automatically skip Claude Code's onboarding screen
by writing hasCompletedOnboarding=true to ~/.claude.json. The setting
defaults to enabled for better user experience.

Cherry-picked from main (ddbff07) with conflict resolution.

tests/msw/handlers.ts

@@ -176,6 +176,10 @@ export const handlers = [
     },
   ),
 
+  http.post(`${TAURI_ENDPOINT}/apply_claude_onboarding_skip`, () => success(true)),
+
+  http.post(`${TAURI_ENDPOINT}/clear_claude_onboarding_skip`, () => success(true)),
+
   http.post(`${TAURI_ENDPOINT}/get_config_dir`, async ({ request }) => {
     const { app } = await withJson<{ app: AppId }>(request);
     return success(app === "claude" ? "/default/claude" : "/default/codex");