feat: search, multi-stop routing, and route display

Full navigation UI with:
- Search bar with 150ms debounced autocomplete from /api/geocode
- Keyboard navigation (arrow keys, Enter, Escape)
- Exact match badge for verified address results
- Multi-stop list with drag-to-reorder (dnd-kit)
- 10-stop cap with disabled state
- Mode selector (drive/walk/bike)
- Valhalla route display with per-leg color polyline
- Maneuver list with instructions, distance, time remaining
- Click maneuver to fly map to that point
- Optimize stops button (3+ stops, uses /optimized_route)
- Responsive: side panel (desktop ≥768px), bottom sheet (mobile)
- Stop pins: green origin, red destination, blue intermediate
- Pin popup with remove button
- Geolocation permission requested on first route, not on load
- Error handling for unroutable pairs
- nginx proxy for /api/ and /valhalla/ endpoints

Dependencies added: zustand, @dnd-kit/core, @dnd-kit/sortable,
@dnd-kit/utilities

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

src/utils/decode.js

@@ -0,0 +1,43 @@
+/**
+ * Decode a Valhalla/Google-encoded polyline string into [lng, lat] coordinate pairs.
+ * Valhalla uses precision 6 by default.
+ * @param {string} encoded
+ * @param {number} precision - decimal precision (6 for Valhalla)
+ * @returns {Array<[number, number]>} Array of [lng, lat] pairs for GeoJSON
+ */
+export function decodePolyline(encoded, precision = 6) {
+  const factor = Math.pow(10, precision)
+  const coords = []
+  let lat = 0
+  let lng = 0
+  let i = 0
+
+  while (i < encoded.length) {
+    let shift = 0
+    let result = 0
+    let byte
+
+    do {
+      byte = encoded.charCodeAt(i++) - 63
+      result |= (byte & 0x1f) << shift
+      shift += 5
+    } while (byte >= 0x20)
+
+    lat += result & 1 ? ~(result >> 1) : result >> 1
+
+    shift = 0
+    result = 0
+
+    do {
+      byte = encoded.charCodeAt(i++) - 63
+      result |= (byte & 0x1f) << shift
+      shift += 5
+    } while (byte >= 0x20)
+
+    lng += result & 1 ? ~(result >> 1) : result >> 1
+
+    coords.push([lng / factor, lat / factor])
+  }
+
+  return coords
+}