Explore places on the map, planner route fixes, and instance-wide Mapbox (#1147)

* feat(maps): add an OSM POI search endpoint (category within a viewport)

New /api/maps/pois queries OpenStreetMap via Overpass for places of a category
(restaurants, cafes, hotels, sights, …) inside a bounding box. OSM-only by design
— it never calls Google, even when a Google key is configured.

* feat(map): explore nearby places on the trip map (OSM category pill)

A floating, icon-only pill over the planner map lets you toggle a POI category and
see those OpenStreetMap places in the current view; clicking a marker opens the
add-place form pre-filled (name, address, website, phone). Single-select with a
'search this area' action after the map moves. Renders on both the Leaflet and
Mapbox maps, and can be turned off in settings (discussion #841).

* fix(planner): anchor timed places when optimising and route transports by location

- The day optimiser no longer reshuffles places that have a set time — they stay
  anchored to their time, like locked places.
- The route now uses a transport's departure/arrival location as a waypoint when it
  has one (e.g. a flight's airport), instead of breaking the route at every booking;
  transports without a location are ignored for routing but still show their leg's
  distance/duration under the booking.

* feat(admin): instance-wide Mapbox defaults in default user settings

Admins can set a shared Mapbox token (plus style, 3D and quality) as instance
defaults, so the whole instance can use Mapbox without each user pasting their own
key. Users without their own value inherit it via the existing admin-defaults
merge; the shared token is stored encrypted (discussion #920).

client/src/components/Map/usePoiExplore.ts

@@ -0,0 +1,76 @@
+import { useState, useRef, useCallback, useMemo } from 'react'
+import { mapsApi } from '../../api/client'
+import type { Poi } from './poiCategories'
+
+export interface Bbox { south: number; west: number; north: number; east: number }
+
+/**
+ * State for the map POI "explore" pill. Toggling a category fetches its OSM POIs
+ * for the current viewport; panning/zooming does NOT auto-refetch — it just marks
+ * the results stale (`moved`) so the pill can offer "search this area". This keeps
+ * Overpass load (and visual churn) down.
+ */
+export function usePoiExplore() {
+  const [active, setActive] = useState<Set<string>>(() => new Set())
+  const [byCat, setByCat] = useState<Record<string, Poi[]>>({})
+  const [loadingKeys, setLoadingKeys] = useState<Set<string>>(() => new Set())
+  const [moved, setMoved] = useState(false)
+
+  const bboxRef = useRef<Bbox | null>(null)
+  // activeRef always mirrors the latest active set so async callbacks (fetch
+  // completions) can check whether a category is still wanted.
+  const activeRef = useRef(active)
+  activeRef.current = active
+
+  const setLoading = useCallback((key: string, on: boolean) => setLoadingKeys(prev => {
+    const next = new Set(prev)
+    if (on) next.add(key); else next.delete(key)
+    return next
+  }), [])
+
+  const fetchCat = useCallback(async (key: string, bbox: Bbox) => {
+    setLoading(key, true)
+    try {
+      const res = await mapsApi.pois(key, bbox)
+      // Drop the result if the user toggled this category off while the (slow)
+      // Overpass request was in flight — otherwise stale results re-appear.
+      setByCat(prev => (activeRef.current.has(key) ? { ...prev, [key]: res.pois } : prev))
+    } catch {
+      setByCat(prev => (activeRef.current.has(key) ? { ...prev, [key]: [] } : prev))
+    } finally {
+      setLoading(key, false)
+    }
+  }, [setLoading])
+
+  const onViewportChange = useCallback((bbox: Bbox) => {
+    bboxRef.current = bbox
+    if (activeRef.current.size > 0) setMoved(true)
+  }, [])
+
+  // Single-select: clicking a category switches to it (dropping the previous one
+  // and its markers immediately) and fetches it for the current viewport; clicking
+  // the already-active category turns it off.
+  const toggle = useCallback((key: string) => {
+    const isOnlyActive = activeRef.current.has(key) && activeRef.current.size === 1
+    setMoved(false)
+    if (isOnlyActive) {
+      setActive(new Set())
+      setByCat({})
+      return
+    }
+    setActive(new Set([key]))
+    setByCat({})
+    if (bboxRef.current) fetchCat(key, bboxRef.current)
+  }, [fetchCat])
+
+  const searchArea = useCallback(() => {
+    const bbox = bboxRef.current
+    if (!bbox) return
+    setMoved(false)
+    activeRef.current.forEach(key => fetchCat(key, bbox))
+  }, [fetchCat])
+
+  const pois = useMemo(() => Object.values(byCat).flat(), [byCat])
+
+  return { active, pois, loadingKeys, moved, toggle, searchArea, onViewportChange }
+}