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/components/Panel.jsx

@@ -1,5 +1,6 @@
 import { useRef, useCallback, useEffect, useState } from 'react'
-import { Sun, Moon, LogIn, LogOut } from 'lucide-react'
+import { Sun, Moon, Sparkles, LogIn, LogOut } from 'lucide-react'
+import { themeList } from '../themes/registry'
 import { useStore, usePanelState } from '../store'
 import { hasFeature } from '../config'
 import SearchBar from './SearchBar'
@@ -54,10 +55,30 @@ export default function Panel({ onManeuverClick }) {
     return () => window.removeEventListener('resize', check)
   }, [])
 
-  // Theme toggle
+  // Theme toggle - cycles through all available themes
   const toggleTheme = () => {
-    const next = theme === 'dark' ? 'light' : 'dark'
-    setThemeOverride(next)
+    const themes = themeList()
+    const currentIdx = themes.findIndex(t => t.id === theme)
+    const nextIdx = (currentIdx + 1) % themes.length
+    setThemeOverride(themes[nextIdx].id)
+  }
+
+  // Get theme icon based on current theme
+  const getThemeIcon = () => {
+    switch (theme) {
+      case 'dark': return <Moon size={16} />
+      case 'light': return <Sun size={16} />
+      case 'clean': return <Sparkles size={16} />
+      default: return <Sun size={16} />
+    }
+  }
+
+  // Get next theme name for tooltip
+  const getNextThemeName = () => {
+    const themes = themeList()
+    const currentIdx = themes.findIndex(t => t.id === theme)
+    const nextIdx = (currentIdx + 1) % themes.length
+    return themes[nextIdx].name
   }
 
   // Auth handlers
@@ -257,10 +278,10 @@ export default function Panel({ onManeuverClick }) {
           onClick={toggleTheme}
           className="p-1.5 rounded"
           style={{ color: 'var(--text-secondary)' }}
-          aria-label={`Switch to ${theme === 'dark' ? 'light' : 'dark'} mode`}
-          title={`Switch to ${theme === 'dark' ? 'light' : 'dark'} mode`}
+          aria-label={`Switch to ${getNextThemeName()} theme`}
+          title={`Switch to ${getNextThemeName()} theme`}
         >
-          {theme === 'dark' ? <Sun size={16} /> : <Moon size={16} />}
+          {getThemeIcon()}
         </button>
       </div>
     </div>