fix: search viewport init, theme-clears-route bug, preview-during-route

Three regressions fixed:

1. mapCenter is now initialized on map 'load' event, not just 'moveend'.
   Searches immediately after page load now correctly include viewport
   bias instead of falling back to default Twin Falls coords.

2. setThemeOverride had stray code from startDirections that wiped
   stops and added undefined place data. Toggling theme cleared the
   active route. Restored setThemeOverride to its correct
   theme-only implementation.

3. usePanelState returned ROUTE_CALCULATED before checking selectedPlace,
   so preview cards could never appear alongside a calculated route.
   Refactored to decouple preview state from route state - preview
   renders whenever selectedPlace exists, independent of route state.

src/components/MapView.jsx

@@ -964,6 +964,13 @@ const MapView = forwardRef(function MapView(_, ref) {
         }
       }
     })
+    // Initialize mapCenter immediately when map loads (Fix 1: search viewport)
+    map.once('load', () => {
+      const center = map.getCenter()
+      const zoom = map.getZoom()
+      setMapCenter({ lat: center.lat, lon: center.lng, zoom })
+    })
+
     map.on('load', () => {
       map.addSource(ROUTE_SOURCE, {
         type: 'geojson',