Add dark mode with system preference auto-detection

All user-facing components now support dark mode via prefers-color-scheme.
- Dark backgrounds: gray-950/900/800
- Dark text: gray-100/200/300/400
- Orange brand colors adapt with darker tints
- Glass effects work in both modes
- Skeletons, cards, filters, bottom sheet all themed
- Google Maps InfoWindow stays light (maps don't support dark)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/BottomSheet.tsx

@@ -89,7 +89,7 @@ export default function BottomSheet({ open, onClose, children }: BottomSheetProp
       {/* Sheet */}
       <div
         ref={sheetRef}
-        className="fixed bottom-0 left-0 right-0 z-50 md:hidden flex flex-col bg-white/85 backdrop-blur-xl rounded-t-2xl shadow-2xl"
+        className="fixed bottom-0 left-0 right-0 z-50 md:hidden flex flex-col bg-white/85 dark:bg-gray-900/90 backdrop-blur-xl rounded-t-2xl shadow-2xl"
         style={{
           height: `${height * 100}vh`,
           transition: dragging ? "none" : "height 0.3s cubic-bezier(0.2, 0, 0, 1)",
@@ -100,7 +100,7 @@ export default function BottomSheet({ open, onClose, children }: BottomSheetProp
       >
         {/* Handle bar */}
         <div className="flex justify-center pt-2 pb-1 shrink-0 cursor-grab">
-          <div className="w-10 h-1 bg-gray-300 rounded-full" />
+          <div className="w-10 h-1 bg-gray-300 dark:bg-gray-600 rounded-full" />
         </div>
 
         {/* Content */}