feat(ui): Redesign bottom-right map control cluster

- Increase touch targets from 36px to 44px (meets accessibility guidelines)
- Wrap Locate and Layers buttons in unified .map-controls-br container
- Layer popover now opens LEFT of buttons (avoids collision with Locate)
- Add hover and active states with theme-aware styling
- Proper spacing for scale control below the cluster
- Increased icon sizes from 18px to 20px
- Mobile-responsive with proper max-height on layer popover

Layout:
  [Locate] 44x44
  [Layers] 44x44
  ──────────────
  Scale: 0.5 mi

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

src/components/LayerControl.jsx

@@ -271,7 +271,7 @@ export default function LayerControl({ mapRef }) {
         title="Map layers"
         aria-label="Toggle map layers"
       >
-        <Layers size={18} />
+        <Layers size={20} />
       </button>
 
       {open && (

src/components/LocateButton.jsx

@@ -48,7 +48,7 @@ export default function LocateButton({ mapRef }) {
       title="My location"
       aria-label="Center map on my location"
     >
-      <Locate size={18} />
+      <Locate size={20} />
     </button>
   )
 }