stop pre-building activity pages to fix OOM build failure

getStaticPaths now returns [] — all /activity/{id}/ URLs are served by
the activity/index.html shell via nginx try_files and hydrated by
ActivityDetailLoader. Pre-rendering thousands of pages was exhausting
server RAM and killing the build. The dynamic loader already handles
public, unlisted, and local activities identically.

site/src/pages/activity/[id].astro

@@ -6,6 +6,15 @@ import ActivityDetail from '../../components/ActivityDetail.svelte';
 import type { BASIndex, ActivitySummary, AthleteZones } from '../../lib/types';
 
 export async function getStaticPaths() {
+  // Activity pages are not pre-built — all /activity/{id}/ URLs are served
+  // by the activity/index.html shell via nginx try_files and loaded dynamically
+  // by ActivityDetailLoader. Pre-building thousands of pages at build time
+  // exhausts server memory. The shell handles public, unlisted, and local
+  // activities identically with no loss of functionality.
+  return [];
+
+  // Dead code below — kept for reference only.
+  /* eslint-disable no-unreachable */
   try {
     const candidates = [
       process.env.BINCIO_DATA_DIR,
@@ -119,6 +128,7 @@ export async function getStaticPaths() {
   } catch {
     return [];
   }
+  /* eslint-enable no-unreachable */
 }
 
 const { activity, athlete } = Astro.props as { activity: ActivitySummary; athlete: AthleteZones | null };