feat(train): a route's map kept offline on request (ADR 0030, D45)

A saved route's page has Keep the map offline: the tiles within 2 km of
its way at zooms 10 to 16 (never above the source's own), shared between
routes, kept again as the route changes and let go with it. The style,
its sprites and the instance's map source are kept as last read, and
every map reads a kept tile before the network, so a map draws with no
network at all.

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

@@ -513,6 +513,18 @@
   "@routeDescent": {"placeholders": {"metres": {"type": "int"}}},
   "routeSummary": "{distance} · {ascent}",
   "@routeSummary": {"description": "A route in the list: its distance and climb.", "placeholders": {"distance": {"type": "String", "example": "12.3 km"}, "ascent": {"type": "String", "example": "120 m up"}}},
+  "routeOffline": "Keep the map offline",
+  "@routeOffline": {"description": "A switch on a saved route: its map kept on this device (ADR 0030, D45)."},
+  "routeOfflineOff": "Its map along the way, 2 km either side, on this device for when there's no network",
+  "routeOfflineLoading": "Keeping the map: {done} of {total} tiles",
+  "@routeOfflineLoading": {"placeholders": {"done": {"type": "int"}, "total": {"type": "int"}}},
+  "routeOfflineKept": "Kept on this device: {size} MB",
+  "@routeOfflineKept": {"placeholders": {"size": {"type": "String", "example": "12.4"}}},
+  "routeOfflineMissing": "{missing} of {total} tiles didn't come: asked for again when the app next opens online",
+  "@routeOfflineMissing": {"placeholders": {"missing": {"type": "int"}, "total": {"type": "int"}}},
+  "routeOfflineNoMap": "The map can't be reached now: try again online",
+  "routeOfflineSaveFirst": "Save the route to keep its map offline",
+  "routeKeptOffline": "Map kept offline",
   "routeSurfaces": "Surfaces",
   "surfacePaved": "Paved",
   "surfaceCompacted": "Compacted",