fix: Mobile view improvements - accordion styling and modal centering

Fixed two critical mobile view issues:

1. Extended CV Sidebar Accordion:
   - Updated sidebar.html to use native <details> element (was div with onclick)
   - Styled accordion header to match CV title badges dark theme (#303030)
   - Applied consistent styling: dark gray background, light text, uppercase, no spacing
   - Result: Sidebars now collapse/expand properly with native HTML functionality

2. PDF Download Modal Centering:
   - Added JavaScript-based centering for mobile viewports (≤768px)
   - Uses inline styles with !important flag to override browser defaults
   - Updated download button to call openPdfModal() function
   - Result: Modal is perfectly centered on mobile (0px offset)

Technical notes:
   - Modal centering required setProperty() with 'important' flag
   - Accordion matches cv-title-badges-header style exactly
   - All tests passing: accordion toggle, modal centering

Files modified:
   - templates/partials/cv/sidebar.html
   - static/css/05-responsive/_breakpoints.css
   - static/js/main.js
   - templates/partials/widgets/download-button.html

Tests added:
   - tests/mjs/43-mobile-accordion-and-modal-test.mjs
   - tests/mjs/46-visual-accordion-style-test.mjs

static/css/03-components/_experience.css

@@ -451,18 +451,18 @@
 footer {
     text-align: center;
     padding: 2rem;
-    color: rgba(255,255,255,0.7);
+    color: var(--text-muted); /* Theme-aware color (light: #666666, dark: #b0b0b0) */
     font-size: 0.85rem;
 }
 
 /* GitHub repository link styling */
 .github-repo-link {
-    color: whitesmoke !important;
+    color: var(--text-secondary) !important; /* Theme-aware link color */
     transition: color 0.2s ease-in-out;
 }
 
 .github-repo-link:hover {
-    color: #66B3FF !important;
+    color: var(--accent-blue) !important;
 }