Use extId station lookup and clamp walk durations

Add ApiClient.findStationByExtId and toStation helper; use it in
useOriginStationWalk. Clamp OSRM route/step durations to a minimum
walking speed (1.25 m/s) and update tests to mock the new API call.

apps/mobile/src/__tests__/screens.test.tsx

@@ -69,6 +69,12 @@ jest.mock('../hooks/useOriginStationWalk', () => ({
 
 jest.mock('../services/api', () => ({
   api: {
+    findStationByExtId: jest.fn().mockResolvedValue({
+      name: 'Mödling Bahnhof',
+      extId: '1231701',
+      lat: 48.085,
+      lng: 16.296,
+    }),
     searchJourneys: jest.fn().mockResolvedValue([
       {
         id: 'journey-1',

apps/mobile/src/hooks/useOriginStationWalk.ts

@@ -24,9 +24,9 @@ export function useOriginStationWalk(origin: Station | null) {
       if (origin?.lat == null || origin.lng == null) return;
 
       try {
-        const nearest = await api.findNearestStationByCoords(origin.lat, origin.lng);
+        const selectedStation = await api.findStationByExtId(origin.extId);
         if (!isMounted) return;
-        setStation(nearest);
+        setStation(selectedStation);
       } catch (err) {
         if (!isMounted) return;
         setLookupError(err instanceof Error ? err.message : 'Origin station lookup failed');