fix: Override .has-tooltip position for theme switcher button

The theme switcher button was hidden above the viewport on desktop because
.has-tooltip's position: relative was overriding the button's position: fixed
due to CSS cascade order (_tooltips.css loaded after _themes.css).

Fixed by adding !important to .color-theme-switcher position: fixed rule.

Changes:
- static/css/01-foundation/_themes.css: Add !important to position: fixed
  to override .has-tooltip position: relative cascade

Testing:
- Added comprehensive tooltip tests for all buttons (desktop & mobile)
- Verified theme switcher visible on desktop at correct position
- Verified all tooltips working on hover (desktop only, hidden on mobile touch)
- Verified button positioning in mobile bottom dock

All buttons now display correctly:
✅ Desktop: All 6 buttons with working tooltips
✅ Mobile: All 5 buttons in bottom dock

static/css/01-foundation/_themes.css

@@ -181,7 +181,7 @@
    THEME SWITCHER BUTTON STYLES - Dynamic colors based on theme mode
    ============================================================================== */
 .color-theme-switcher {
-  position: fixed;
+  position: fixed !important; /* Override .has-tooltip position: relative */
   bottom: 14rem; /* Middle position - between print (18rem) and shortcuts (10rem) */
   left: 2rem;
   width: 50px;