feat: Store API key locally in browser instead of build-time

- Add apiKeyService for localStorage-based API key management
- Add ApiKeyModal component for entering/updating API key
- Update geminiService to use dynamic API key
- Remove .env.example as API key is now user-provided
- Update to gemini-2.0-flash model

The API key is now stored only in the user's browser localStorage,
making the app more secure and easier to deploy.

https://claude.ai/code/session_01DBAyjuKW8Qtzixc64qn9KP

services/apiKeyService.ts

@@ -0,0 +1,18 @@
+const STORAGE_KEY = 'gemini_api_key';
+
+export const getApiKey = (): string | null => {
+  return localStorage.getItem(STORAGE_KEY);
+};
+
+export const setApiKey = (key: string): void => {
+  localStorage.setItem(STORAGE_KEY, key);
+};
+
+export const clearApiKey = (): void => {
+  localStorage.removeItem(STORAGE_KEY);
+};
+
+export const hasApiKey = (): boolean => {
+  const key = getApiKey();
+  return key !== null && key.length > 0;
+};