UI-02: animated skeleton map when tiles can't be fetched

Adds MapConnectivityState, a shared tracker of tile-fetch outcomes (cache
miss + network failure) that flips every map into an animated skeleton
after 3 consecutive failures and recovers on a single success -- either an
ordinary fetch succeeding, or (once TileLayer has been fully unmounted in
skeleton mode) a periodic single-tile probe every 15s. Detected at the
fetch level rather than via an OS connectivity API, since a captive portal
or degraded connection can report "online" while every real fetch times
out.

SkeletonMapLayer reuses the Stitch exports' 40px grid-overlay treatment
with a shimmer sweep, replacing TileLayer entirely (never fetching
underneath its own placeholder) while markers/polylines keep rendering
since they come from local data. RideMap and the route planner's
independent FlutterMap both wire this in via a plain skeletonMode bool.

Moved the tile-source constants into a new tiles/tile_config.dart so the
connectivity probe (in the app-layer composition root) doesn't need to
import from ui/ to build its request URL.

Verified end-to-end on a real emulator: cut network, cleared the tile
cache, confirmed the skeleton renders after real fetch failures, then
confirmed automatic recovery within one probe interval once network
returned -- not just via the widget/unit tests that also cover this.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012Xki7YAcc2TiN2PRZJ2tXr

lib/src/ui/app_shell.dart

@@ -94,6 +94,7 @@ class ShellScaffold extends ConsumerWidget {
                   fill: true,
                   showEmptyLabel: false,
                   tileProvider: ref.watch(cachedTileProviderProvider),
+                  skeletonMode: ref.watch(mapConnectivityProvider).skeletonMode,
                 ),
               ),
             ),