fix(hikes): sync tag filter to URL + re-fit overview map on filter change

Two bugs:

* Toggling a tag in the filter bar didn't update `?tag=` in the URL,
  so the page wasn't shareable / back-button-restorable past the
  initial deep-link state. Add a writer $effect that mirrors
  `filter.tags` into the URL via `replaceState` (no history churn).

* The overview map's polylines + camera were built once on mount and
  never refreshed when the `hikes` prop changed, so filtering left
  the map showing the full set and zoomed for it. Extract polyline
  rendering into a function called both on mount and from a
  prop-watching $effect; on change, smoothly fly to the new union.

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "homepage",
-	"version": "1.75.3",
+	"version": "1.75.4",
 	"private": true,
 	"type": "module",
 	"scripts": {