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": {

src/lib/components/hikes/HikesOverviewMap.svelte

@@ -185,9 +185,15 @@
 			});
 
 			// One polyline per hike, sourced from the manifest's already-
-			// simplified previewPolyline (≤150 points each).
+			// simplified previewPolyline (≤150 points each). The layer is
+			// re-populated on every `hikes` prop change (see the $effect
+			// below) so toggling filters updates the visible routes — and
+			// re-fits the camera to the new union bounds.
 			const layer = L.layerGroup().addTo(map);
-			const bounds = L.latLngBounds([]);
+
+			function renderPolylines(): boolean {
+				layer.clearLayers();
+				const b = L.latLngBounds([]);
 				for (const hike of hikes) {
 					if (!hike.previewPolyline || hike.previewPolyline.length < 2) continue;
 					const latLngs = hike.previewPolyline.map(([lat, lng]) => [lat, lng] as [number, number]);
@@ -216,20 +222,11 @@
 					});
 
 					for (const [lat, lng] of latLngs) {
-					bounds.extend([lat, lng]);
+						b.extend([lat, lng]);
 					}
 				}
-
-			if (bounds.isValid()) {
-				initialBounds = bounds;
-				// When the caller handed us a pre-rendered hero pose, we
-				// already called `setView(initialCenter, initialZoom)` above
-				// and rely on the tile-load handler to fly to bounds (so the
-				// static→live cross-fade happens at the matching pose). With
-				// no pre-rendered hero, fitBounds straight away.
-				if (!initialCenter || typeof initialZoom !== 'number') {
-					map.fitBounds(initialBounds, { padding: [32, 32], maxZoom: 13 });
-				}
+				if (b.isValid()) {
+					initialBounds = b;
 					recenterMap = () => {
 						if (!initialBounds) return;
 						map.flyToBounds(initialBounds, {
@@ -239,6 +236,18 @@
 							easeLinearity: 0.25
 						});
 					};
+					return true;
+				}
+				initialBounds = null;
+				recenterMap = null;
+				return false;
+			}
+
+			// Initial paint — no animated fit when the caller handed us a
+			// pre-rendered hero pose (the tile-load handover handles the
+			// fly-to), otherwise fit straight to the union bounds.
+			if (renderPolylines() && (!initialCenter || typeof initialZoom !== 'number') && initialBounds) {
+				map.fitBounds(initialBounds, { padding: [32, 32], maxZoom: 13 });
 			}
 
 			// User location (opt-in). Same Tauri-first / Web-Geolocation-fallback
@@ -300,6 +309,26 @@
 
 			// React to control toggles outside the attachment.
 			const stopReactRoot = $effect.root(() => {
+				// Re-render polylines whenever the `hikes` prop changes
+				// (filter bar toggles, tag deep-link). The first $effect
+				// run fires immediately and would re-do the initial paint
+				// for no UX gain — skip it via a tick counter.
+				let rerunTick = 0;
+				$effect(() => {
+					void hikes;
+					if (rerunTick++ === 0) return;
+					if (renderPolylines() && initialBounds) {
+						// Smooth re-fit so the user sees the camera glide
+						// toward whichever subset is now on display.
+						map.flyToBounds(initialBounds, {
+							padding: [32, 32],
+							maxZoom: 13,
+							duration: 0.6,
+							easeLinearity: 0.25
+						});
+					}
+				});
+
 				$effect(() => {
 					if (baseLayer === currentBase) return;
 					tileLayers[currentBase].remove();

src/routes/hikes/+page.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
 	import { SvelteSet } from 'svelte/reactivity';
 	import { page } from '$app/state';
+	import { replaceState } from '$app/navigation';
 	import HikeCard from '$lib/components/hikes/HikeCard.svelte';
 	import HikesFilterBar, { type HikesFilter } from '$lib/components/hikes/HikesFilterBar.svelte';
 	import HikesOverviewMap from '$lib/components/hikes/HikesOverviewMap.svelte';
@@ -60,14 +61,36 @@
 	});
 
 	// Tag deep-link: arrival from a detail-page tag chip (`/hikes?tag=winter`)
-	// or any saved URL with `?tag=...` pre-selects those tags. Repeated
-	// params accumulate (`?tag=winter&tag=easy`). Only runs on the client —
-	// SSR has no searchParams to read here.
+	// or any saved URL with `?tag=...` pre-selects those tags. Runs once on
+	// mount; thereafter the URL writer below is the source of truth.
+	let initialTagsApplied = false;
 	$effect(() => {
+		if (initialTagsApplied) return;
 		if (typeof window === 'undefined') return;
 		const params = page.url.searchParams.getAll('tag');
-		if (params.length === 0) return;
 		for (const t of params) if (t) filter.tags.add(t);
+		initialTagsApplied = true;
+	});
+
+	// Tag URL sync: every toggle in the filter bar reflects into the URL
+	// so the page is shareable / back-button-restorable. `replaceState`
+	// rather than `goto` keeps history clean — toggling four tags would
+	// otherwise leave four back-button stops.
+	$effect(() => {
+		if (typeof window === 'undefined' || !initialTagsApplied) return;
+		const url = new URL(window.location.href);
+		const wanted = [...filter.tags].sort();
+		const current = url.searchParams.getAll('tag').slice().sort();
+		// Skip the no-op rewrite path — `replaceState` would still touch
+		// history's state object and trigger downstream `page.url` effects
+		// for no UX benefit.
+		if (
+			wanted.length === current.length &&
+			wanted.every((t, i) => t === current[i])
+		) return;
+		url.searchParams.delete('tag');
+		for (const t of wanted) url.searchParams.append('tag', t);
+		replaceState(url, page.state);
 	});
 
 	// One-shot per mount: set the slider ceilings to the actual data maxes.