Fix site loading: broken CSS ref, wrong Gemini model, missing Pages config docs - #2

The site failed to deploy because GitHub Pages was never enabled, and contained several code bugs that would cause runtime failures once deployed.

Root Cause

The GitHub Actions deploy job exited with HTTP 404 — actions/deploy-pages requires Settings → Pages → Source → GitHub Actions to be enabled first. Added setup instructions to README.

Code Fixes

• index.html: Removed <link rel="stylesheet" href="/index.css"> — file doesn't exist, causing a Vite build warning and a 404 at runtime
• geminiService.ts: Replaced non-existent model gemini-3-flash-preview with gemini-2.0-flash; removed the unsupported thinkingConfig block; dropped unused Type import
• geminiService.ts: Aligned env var reference from process.env.API_KEY → process.env.GEMINI_API_KEY to match Vite config and .env.local convention

// Before
import { GoogleGenAI, Type } from "@google/genai";
const ai = new GoogleGenAI({ apiKey: process.env.API_KEY });
// model: 'gemini-3-flash-preview', thinkingConfig: { thinkingBudget: 0 }

// After
import { GoogleGenAI } from "@google/genai";
const ai = new GoogleGenAI({ apiKey: process.env.GEMINI_API_KEY });
// model: 'gemini-2.0-flash'

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