style: Add CSS variable fallbacks for better browser compatibility

Following LogRocket CSS best practices:
- Added fallback values to 150+ CSS variable usages across 22 files
- Fallbacks use light theme defaults for consistent behavior
- Improves compatibility with older browsers
- Example: var(--text-primary) → var(--text-primary, #1a1a1a)

Variables with fallbacks:
- Colors: text-primary, text-secondary, text-muted, accent-blue, etc.
- Backgrounds: page-bg, paper-bg, action-bar-bg, sidebar-bg
- Shadows: shadow-sm, shadow-md, shadow-lg
- Borders: border-color, border-light, icon-border

CSS Variables Best Practices compliance: 6/7 recommendations now followed

static/css/02-layout/_grid.css

@@ -33,6 +33,6 @@
 
 /* Main Content Area */
 .cv-main {
-  background: var(--paper-bg);
+  background: var(--paper-bg, #ffffff);
   padding: 3rem 2.5rem 8rem 2.5rem; /* Bottom padding for footer and zoom control clearance */
 }