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 */

static/css/01-foundation/_themes.css

@@ -186,7 +186,7 @@
   left: 2rem;
   width: 50px;
   height: 50px;
-  background: var(--black-bar);
+  background: var(--black-bar, #2b2b2b);
   color: white;
   border: none;
   border-radius: 50%;
@@ -348,6 +348,6 @@
 @media (prefers-color-scheme: light) {
   [data-color-theme="auto"] img[src*="livgolf"] {
     filter: none;
-    border-color: var(--icon-border) !important; /* Reset to normal border */
+    border-color: var(--icon-border, #ddd) !important; /* Reset to normal border */
   }
 }

static/css/01-foundation/_typography.css

@@ -7,7 +7,7 @@
 /* Base Typography */
 body {
   font-family: 'Quicksand', 'Source Sans Pro', -apple-system, system-ui, sans-serif;
-  color: var(--text-secondary);
+  color: var(--text-secondary, #333333);
   line-height: 1.5;
   font-size: 16px;
   font-weight: 400;
@@ -18,7 +18,7 @@ body {
 
 /* Links */
 a {
-  color: var(--accent-blue);
+  color: var(--accent-blue, #0066cc);
   text-decoration: none;
 
   &:hover {