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/astro.config.mjs

@@ -9,15 +9,20 @@ const env = loadEnv(process.env.NODE_ENV ?? 'development', process.cwd(), '');
 const apiPort = process.env.VITE_API_PORT || '4041';
 const serveTarget = env.PUBLIC_EDIT_URL || `http://localhost:${apiPort}`;
 
-// In production, nginx serves activity/index.html for all /activity/<id>/ URLs
-// via try_files. In dev (Astro dev server), no nginx — this plugin replicates it.
-const activityFallbackPlugin = {
-  name: 'activity-shell-fallback',
+// In production, nginx serves the shell for dynamic sub-paths via try_files.
+// In dev (Astro dev server), no nginx — this plugin replicates those rules.
+const shellFallbackPlugin = {
+  name: 'shell-fallback',
   configureServer(server) {
     server.middlewares.use((req, _res, next) => {
       if (req.url && /^\/activity\/[^/]+\/?(\?|$)/.test(req.url)) {
         req.url = '/activity/';
       }
+      // /segments/{id}/ → /segments/  (but not /segments/new/ which has its own page)
+      const segMatch = req.url?.match(/^\/segments\/([^/?]+)\//);
+      if (segMatch && segMatch[1] !== 'new') {
+        req.url = '/segments/';
+      }
       next();
     });
   },
@@ -30,7 +35,7 @@ export default defineConfig({
   // When hosting at a subdirectory (e.g. GitHub Pages project site), set:
   // base: "/repo-name",
   vite: {
-    plugins: [activityFallbackPlugin],
+    plugins: [shellFallbackPlugin],
     optimizeDeps: {
       include: ['maplibre-gl'],
       esbuildOptions: { target: 'es2022' },