fix(admin): handle /admin directory vs /admin/files correctly

/admin path (no trailing slash or ext) must serve admin.html SPA,
/admin/dashboard.html must serve ./public/admin/dashboard.html.
Previously /admin tried to read ./public/admin directory causing 500.

Refs: production server

src/server/index.ts

@@ -1803,7 +1803,9 @@ app.get('/src/i18n/*', serveStatic({ root: '.' }))
 
 // Admin HTML components - read file text explicitly to force Content-Length
 app.get('/admin/*', async (c) => {
-  const filePath = `./public${c.req.path}`
+  const path = c.req.path
+  // If path ends with /, it's a directory → serve admin SPA
+  const filePath = path.endsWith('/') ? './public/admin.html' : `./public${path}`
   const file = Bun.file(filePath)
   const text = await file.text()
   c.header('Content-Length', String(Buffer.byteLength(text, 'utf8')))