Fix stuck segments tab; add /segments/ dev fallback

AthleteView: use segmentsFetched flag to prevent infinite fetch loop when
there are no efforts (segmentSummary.length === 0 was re-triggering the
reactive statement after every empty response). Also improve empty state
message and reset flag after rescan so the table reloads.

astro.config.mjs: extend shell fallback plugin to cover /segments/{id}/
the same way /activity/{id}/ is handled, so segment detail pages work in
the dev server without nginx.

site/src/components/AthleteView.svelte

@@ -32,6 +32,7 @@
   }
   let segmentSummary: SegmentSummaryItem[] = [];
   let segmentsLoading = false;
+  let segmentsFetched = false;
   let segmentsHandle = '';
   let rescanning = false;
   let rescanMsg: string | null = null;
@@ -46,8 +47,9 @@
     history.replaceState(null, '', qs ? `?${qs}` : window.location.pathname);
   }
 
-  $: if (activeTab === 'segments' && segmentsHandle && segmentSummary.length === 0 && !segmentsLoading) {
+  $: if (activeTab === 'segments' && segmentsHandle && !segmentsFetched && !segmentsLoading) {
     segmentsLoading = true;
+    segmentsFetched = true;
     fetch(`/api/users/${segmentsHandle}/segment_summary`)
       .then(r => r.ok ? r.json() : [])
       .then(d => { segmentSummary = d; })
@@ -191,8 +193,8 @@
               const d = await r.json();
               if (r.ok) {
                 rescanMsg = `Found ${d.efforts_found} effort${d.efforts_found !== 1 ? 's' : ''}.`;
-                // Refresh the summary
                 segmentSummary = [];
+                segmentsFetched = false;
               } else rescanMsg = d.detail ?? 'Rescan failed.';
             } catch { rescanMsg = 'Could not reach server.'; }
             rescanning = false;
@@ -203,9 +205,9 @@
       </div>
     </div>
     {#if segmentsLoading}
-      <p class="text-zinc-500 text-sm">Loading segments…</p>
+      <p class="text-zinc-500 text-sm">Loading…</p>
     {:else if segmentSummary.length === 0}
-      <p class="text-zinc-500 text-sm">No segment efforts yet.</p>
+      <p class="text-zinc-500 text-sm">No segment efforts yet. Use "Rescan all activities" to detect efforts from existing activities.</p>
     {:else}
       <div class="bg-zinc-900 rounded-xl border border-zinc-800 overflow-hidden">
         <table class="w-full text-sm">