Follow mode (explicit Start; off by default — a watcher drains battery):
watchPosition in geolocation.ts, live dot, 'NEXT · step N' banner with
distance-to-turn + distance-to-go, current step highlighted, map follows.
A transient watch error (timeout/unavailable) no longer kills it — only a
permission denial does; the watcher recovers on its own.
Choose your start: 'From → My location / Choose on map' — tapping the map sets
the origin, and the route recomputes from there.
Offline: src/lib/routeCache.ts keeps the last route per destination+mode with
the line downsampled to <=250 points. If a fetch fails (no signal / routing
server down) the dialog falls back to it with an 'Offline · saved route {age}'
badge instead of a dead error.
Also tidied the map overlays (offline badge bottom-left, Whole route
bottom-right) so nothing covers Leaflet's zoom control.
Verified with CDP: controls present; follow banner tracks movement (step 1 ->
step 7, distances update); blocking routing shows the cached route + badge;
picking a start on the map changed the route (968 m -> 922 m). Still no voice
guidance or re-routing if you leave the route (documented).
