feat(themes): add Clean theme — Google Maps-inspired style

Add a plain, utilitarian theme focused on readability and wayfinding:
- White/light gray land (#f5f5f5), soft pastel green parks (#c3ecb2)
- Gentle blue water (#aadaff) with classic road hierarchy
- White minor roads, yellow primary (#fbc02d), orange motorway (#f9a825)
- Pure white UI panels with Google-standard gray text
- All 73 protomaps flavor keys + pois + landcover objects
- Full UI CSS custom properties using Google color palette
- Overlay config for hillshade, contours, public lands, trails

Update theme switcher to cycle through all available themes.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/themes/registry.js

@@ -14,7 +14,8 @@
  *   ui: object        - CSS custom properties for UI elements
  */
 
-import { namedTheme } from 'protomaps-themes-base'
+import { namedTheme } from 'protomaps-themes-base'
+import cleanTheme from './clean.js'
 
 // ═══════════════════════════════════════════════════════════════════════════
 // UI CSS CUSTOM PROPERTIES
@@ -441,6 +442,7 @@ const themes = {
     overlay: darkOverlay,
     ui: darkUI,
   },
+  clean: cleanTheme,
   // Custom themes go here. Example:
   // 'midnight': {
   //   id: 'midnight',