feat(train): search for places in the route planner, through Photon (ADR 0030, D50)

A `search` endpoint asks the instance's Photon (compose.search.yaml, Europe's
database from GraphHopper, renewed monthly) for people signed in, limited and
unlogged. The planner gets a field over its map: the places found nearest the
map's centre first, in the app's language; one chosen is the next waypoint.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/train/halkyon_train_flutter/lib/l10n/app_en.arb

@@ -503,6 +503,13 @@
   "routeProfileMtb": "Mountain bike",
   "routeTapHint": "Tap the map to add a waypoint; hold one and drag it to move it.",
   "routeUndo": "Take the last waypoint off",
+  "routeSearchHint": "Search for a place",
+  "@routeSearchHint": {"description": "The planner's search field over the map (D50)."},
+  "routeSearchClear": "Clear the search",
+  "routeSearchNothing": "Nothing found.",
+  "@routeSearchNothing": {"description": "Under the planner's search: the instance found no place."},
+  "routeSearchTooOften": "Too many searches for now; try again in a minute.",
+  "routeSearchUnreachable": "The search can't be reached now.",
   "routePlanning": "Planning…",
   "routeRefusedInvalid": "A waypoint too many, or one off the map.",
   "routeRefusedTooOften": "Planned too often: try again in a minute.",