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/_container.css

@@ -22,7 +22,7 @@
     transition: all 0.3s ease-in-out;
 
     .cv-page {
-      box-shadow: var(--shadow-lg);
+      box-shadow: var(--shadow-lg, 2px 2px 9px rgba(0,0,0,0.5));
       border: none;
       margin: 0 auto;
       max-width: 900px;

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 */
 }

static/css/02-layout/_page.css

@@ -1,8 +1,8 @@
 .cv-page {
-    background: var(--paper-bg);
+    background: var(--paper-bg, #ffffff);
     max-width: 1200px;
     margin: 2rem auto;
-    box-shadow: var(--shadow-lg);
+    box-shadow: var(--shadow-lg, 2px 2px 9px rgba(0,0,0,0.5));
     border: none;
     transform: scale(0.95);
     transform-origin: top center;