feat(hikes): clickable tag chips + tag filter on the overview

Detail-page tag chips become anchor links to `/hikes?tag=<name>`.
HikesFilterBar grows a tags fieldset (sorted by frequency, with the
hash prefix the chips use) so the user can keep narrowing from there.

Multi-tag filtering is OR — a hike matching any selected tag stays
visible. AND would shrink the listing fast given how few tags most
hikes carry; OR matches what "show me more like this" feels like.

The overview page reads `tag` query params on mount and pre-fills the
filter — supports repeated params (`?tag=winter&tag=easy`).

src/routes/hikes/[slug]/+page.svelte

@@ -424,10 +424,14 @@
 	{#if hike.tags.length > 0}
 		<!-- Tag chips sit between the metric tiles (facts) and the
 		     elevation profile (data viz) so they read as framing context —
-		     "what kind of hike is this" — before the data takes over. -->
+		     "what kind of hike is this" — before the data takes over.
+		     Each chip is an anchor link to the /hikes overview with that
+		     tag pre-selected in the filter bar. -->
 		<section class="tags" aria-label="Schlagwörter">
 			{#each hike.tags as tag (tag)}
-				<span class="tag-chip"><span class="tag-hash" aria-hidden="true">#</span>{tag}</span>
+				<a class="tag-chip" href="/hikes?tag={encodeURIComponent(tag)}">
+					<span class="tag-hash" aria-hidden="true">#</span>{tag}
+				</a>
 			{/each}
 		</section>
 	{/if}
@@ -768,6 +772,17 @@
 		background: var(--color-bg-tertiary);
 		border-radius: var(--radius-pill);
 		letter-spacing: 0.005em;
+		text-decoration: none;
+		transition:
+			color var(--transition-fast),
+			background-color var(--transition-fast),
+			scale var(--transition-fast);
+	}
+
+	.tag-chip:hover {
+		color: var(--color-primary);
+		background: var(--color-bg-elevated);
+		scale: 1.05;
 	}
 
 	.tag-hash {
@@ -775,6 +790,10 @@
 		font-weight: 600;
 	}
 
+	.tag-chip:hover .tag-hash {
+		color: color-mix(in oklab, var(--color-primary) 60%, currentColor);
+	}
+
 	.elev-area {
 		padding: 0 1rem;
 		margin-top: 0.25rem;