Migrate to @google/genai SDK and fix Gemini API test

- Replace legacy @google/generative-ai with new @google/genai SDK
- Add model fallback chain: gemini-3.5-flash -> gemini-3-flash-preview -> gemini-2.5-flash
- Fix onboarding API key test checking wrong property (result.success vs result.ok)
- Improve 503/high-demand and quota error messages
- Fix packaged app icon paths using __dirname
- Unpack icons and prompts from asar so they load in packaged builds

Author: TechyCSR

main.js

@@ -1551,7 +1551,7 @@ class ApplicationController {
         return { success: false, error: "Invalid icon key" };
       }
 
-      const fullIconPath = path.resolve(iconPath);
+      const fullIconPath = path.resolve(__dirname, iconPath);
 
       if (!fs.existsSync(fullIconPath)) {
         logger.error("Icon file not found", {
@@ -1636,7 +1636,7 @@ class ApplicationController {
           // Force dock refresh
           setTimeout(() => {
             app.dock.setIcon(
-              require("path").resolve(`assests/icons/${iconKey}.png`)
+              require("path").resolve(__dirname, `assests/icons/${iconKey}.png`)
             );
           }, 50);
         }

onboarding.js

@@ -198,7 +198,7 @@
       // saveSettings writes the key into .env (atomic via persistEnvUpdates).
       await window.electronAPI.saveSettings({ geminiKey: key });
       const result = await window.electronAPI.testGeminiConnection();
-      if (result && result.ok) {
+      if (result && result.success) {
         state.geminiKey = key;
         setKeyStatus('success', `Connected — ${result.model || 'Gemini ready'}`);
       } else {