fix: Navigation menu text colors in dark theme

The menu was showing light gray text in dark theme, creating poor contrast
against the white menu background. Menu text must always be dark since
the menu background is always white regardless of theme.

Changes:
- Added dark theme overrides for .navigation-menu and .submenu-content
- Force --text-dark to #1a1a1a (dark text) in dark theme for menu
- Force --text-gray to #333333 (dark gray) in dark theme for menu icons
- Applied same fix for auto theme when system preference is dark

Result:
- Menu text: Dark/black (rgb(26, 26, 26)) in all themes
- Menu icons: Dark gray (rgb(51, 51, 51)) in all themes
- Menu background: White (rgb(255, 255, 255)) in all themes
- Proper contrast and readability restored

Test created: 68-menu-colors-dark-theme-test.mjs
Screenshots: menu-light-theme.png, menu-dark-theme.png

static/css/04-interactive/_navigation.css

@@ -355,3 +355,24 @@ html {
     }
 }
 
+/* ==============================================================================
+   DARK THEME - Menu Text Colors Override
+   ============================================================================== */
+/* Navigation menu always has white background, so text must always be dark */
+[data-color-theme="dark"] .navigation-menu,
+[data-color-theme="dark"] .navigation-menu .submenu-content {
+    /* Menu items */
+    --text-dark: #1a1a1a;  /* Force dark text for white menu background */
+    --text-gray: #333333;  /* Force dark gray for icons on white background */
+}
+
+/* Auto theme - dark mode */
+@media (prefers-color-scheme: dark) {
+    [data-color-theme="auto"] .navigation-menu,
+    [data-color-theme="auto"] .navigation-menu .submenu-content {
+        /* Menu items */
+        --text-dark: #1a1a1a;  /* Force dark text for white menu background */
+        --text-gray: #333333;  /* Force dark gray for icons on white background */
+    }
+}
+