feat: add persisted manual pi backend mode controls

src/preferences.test.ts

@@ -30,6 +30,12 @@ describe('preferences', () => {
     expect(prefs.modelTier).toBe('local');
   });
 
+  it('round-trips backend mode preference', () => {
+    savePreference(dataDir, 'backendMode', 'force_native');
+    const prefs = loadPreferences(dataDir);
+    expect(prefs.backendMode).toBe('force_native');
+  });
+
   it('merges preferences without overwriting other keys', () => {
     savePreference(dataDir, 'modelTier', 'fast');
     savePreference(dataDir, 'otherKey', 42);
@@ -43,4 +49,14 @@ describe('preferences', () => {
     savePreference(nested, 'modelTier', 'default');
     expect(loadPreferences(nested).modelTier).toBe('default');
   });
+
+  it('ignores invalid backendMode values in stored preferences', () => {
+    writeFileSync(
+      resolve(dataDir, 'preferences.json'),
+      JSON.stringify({ modelTier: 'default', backendMode: 'invalid-mode' }),
+    );
+    const prefs = loadPreferences(dataDir);
+    expect(prefs.modelTier).toBe('default');
+    expect(prefs.backendMode).toBeUndefined();
+  });
 });