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/01-foundation/_reset.css

@@ -11,7 +11,7 @@
 
 /* Body base */
 body {
-  background-color: var(--page-bg);
+  background-color: var(--page-bg, #d6d6d6);
 
   /* OLD PATTERN - Keep for reference (can be restored anytime) */
   /* background-image:
@@ -22,7 +22,7 @@ body {
   background-size: 50px 50px, 50px 50px, 10px 10px, 10px 10px; */
 
   /* NEW TEST PATTERNS - Theme-specific (woven fabric for light, diagonal grid for dark) */
-  background-image: var(--page-bg-pattern);
+  background-image: var(--page-bg-pattern, none);
   background-size: 40px 40px; /* For dark theme diagonal grid */
   background-attachment: fixed;
   max-width: 100vw; /* Prevent horizontal overflow */