Fix device selection default mismatch + Integrate Waterfall & Comparison

Bug Fix:
- Backend was defaulting to mobile (isMobile ?? true)
- Frontend was defaulting to desktop
- Changed runner.js default to false (desktop) to match frontend

Feature Integration:
- Added Waterfall button to each history item
- Added comparison checkboxes to history for easy test selection
- Auto-limit to 2 tests for comparison
- Comparison controls appear dynamically when tests selected
- Auto-load comparison when URL params present

lib/runner.js

@@ -58,7 +58,7 @@ async function _executeTest(url, options) {
   const chromeLauncher = await import('chrome-launcher');
   const { v4: uuidv4 } = await import('uuid');
 
-  const isMobile = options.isMobile ?? true; // Default to mobile
+  const isMobile = options.isMobile ?? false; // Default to desktop (matches frontend)
   const reportDir = path.join(__dirname, '..', 'reports');
 
   // Ensure reports directory exists