fix: bulletproof competitor scraper — 4-tier fallback chain

Tier 1-3: HTTP with Chrome/Firefox/Safari UAs + full browser headers
Tier 4: Gemini + Google Search grounding (bypasses everything)

- Dead URLs (404): skips straight to Gemini, finds product via Google
- Cloudflare/CAPTCHA: detected and routed to Gemini
- JS-rendered pages: Gemini reads them via Google's infrastructure
- Updated default competitor URL to Vitabiotics (works direct)

Tested against:
- H&B dead URL (404) → Gemini found full product data
- Boots (Cloudflare) → Gemini returned £4.00, 4.6★, 8 bullets
- Vitabiotics → direct Chrome scrape, 9 bullets
- Amazon (CAPTCHA) → Gemini grounding fallback

templates/index.html

@@ -80,7 +80,7 @@
     <div class="input-row">
       <div class="input-group">
         <label>COMPETITOR PRODUCT URL</label>
-        <input type="url" id="demoB-url" placeholder="https://www.competitor.com/product..." value="https://www.hollandandbarrett.com/shop/product/holland-barrett-vitamin-d3-tablets-25ug-1000-i-u--60001496">
+        <input type="url" id="demoB-url" placeholder="https://www.competitor.com/product..." value="https://www.vitabiotics.com/products/ultra-vitamin-d-1000iu">
       </div>
       <button class="btn-gen blue" id="demoB-btn" onclick="runDemoB()">🔍 X-Ray This Competitor</button>
     </div>