feat: Refactor Gemini integration and build process

This commit centralizes Gemini API interaction into a new `server.ts` file and removes it from the client-side `geminiService.ts`.
It also updates the build process in `package.json` to use `esbuild` for bundling the server code, enabling a more robust build pipeline.
The `index.html` has been cleaned up by removing unnecessary OneSignal SDK initialization.
Additionally, dev dependencies have been updated, including `esbuild` to version `0.28.0`.
A new `test2.ts` file is added for testing Gemini API connectivity.

test2.ts

@@ -0,0 +1,18 @@
+import { GoogleGenAI } from "@google/genai";
+import * as dotenv from "dotenv";
+dotenv.config();
+
+const test = async () => {
+  const ai = new GoogleGenAI({});
+  try {
+    const response = await ai.models.generateContent({
+      model: "gemini-3.1-pro-preview",
+      contents: "Tell me a joke"
+    });
+    console.log("Success:", response.text);
+  } catch (err: any) {
+    console.error("Error:", err.message);
+  }
+}
+
+test();