Tap the destination in the header to search and change where you're going:
- New DestinationSearch: searches skate spots, skate shops and BTCMap merchants
(merchants come from a radius around the current destination - BTCMap has no
global name search - and are labelled as such). Mounted only while the search
is open, so opening Directions pays nothing for it; spot/shop limits match the
map page so the cache is reused.
- RouteDialog now owns the destination internally (seeded from the prop on open,
not on every render) so the search can change it without re-opening the dialog,
and calls onDestinationChange so the parent stays in sync (wired in MapPage).
- The Leaflet map is no longer rebuilt when the destination changes: it's created
from a ref once per open, and a small effect just moves the destination marker
(the route effect re-fits the view). This avoids a teardown/recreate flash.
