feat(dayplan): offer public transport on any connector leg (#1281 follow-up)

The per-segment travel-mode menu now carries a "Public transit" entry on every
editable connector: stop-to-stop legs and the boundary hotel/booking bookends
alike. Choosing it opens the existing transit search pre-filled with the leg's
origin, destination and the stop's departure time, instead of drawing an OSRM
road route.

A connector only renders when both endpoints are located, so the option is
inherently limited to legs that can actually be searched. A leg's coordinates
are resolved back to named, timed points from the trip's places, accommodations
and booking endpoints. TransitSearchPanel gains an initialTime prop so the
departure time can be seeded.

client/src/components/Planner/TransitSearchPanel.test.tsx

@@ -177,6 +177,20 @@ describe('TransitSearchPanel', () => {
     expect(transitApiMock.plan.mock.calls[0][0]).toMatchObject({ arriveBy: true, time: '2025-06-01T08:00:00.000Z' })
   })
 
+  it('FE-PLANNER-TRANSIT-008b: initialTime seeds the departure time', async () => {
+    const user = userEvent.setup()
+    transitApiMock.plan.mockResolvedValueOnce({ itineraries: [] })
+    render(<TransitSearchPanel {...makeProps({
+      initialFrom: { name: 'Fernsehturm', lat: 52.5208, lng: 13.4094 },
+      initialTo: { name: 'London Victoria', lat: 51.5074, lng: -0.1278 },
+      initialTime: '11:00',
+    })} />)
+    await user.click(screen.getByRole('button', { name: /^Search$/ }))
+    await waitFor(() => expect(transitApiMock.plan).toHaveBeenCalled())
+    // 11:00 in Berlin (UTC+2) anchors depart-by to the origin zone → 09:00Z.
+    expect(transitApiMock.plan.mock.calls[0][0]).toMatchObject({ time: '2025-06-01T09:00:00.000Z' })
+  })
+
   it('FE-PLANNER-TRANSIT-009: arrive-by lists the latest-arriving itinerary first', async () => {
     const user = userEvent.setup()
     // MOTIS returns arrive-by results ascending, deadline-adjacent last (#1479).