fix(dashboard): serve embedded pages via io.ReadSeeker (fixes 500 on /fleet)

serveEmbeddedFile asserted the embedded file implemented
`interface { Len() int64; io.ReadSeeker }`, but *embed.openFile has no
Len() method, so http.ServeContent panicked (caught by chi Recoverer ->
500) on every embedded page: /fleet, /ambient, /live, /setup, /simple, /.
http.ServeContent only needs an io.ReadSeeker, which embed files satisfy.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

VERSION

@@ -1 +1 @@
-0.1.352
+0.1.353

mothership/cmd/mothership/main.go

@@ -311,10 +311,12 @@ func serveEmbeddedFile(w http.ResponseWriter, r *http.Request, filename string)
 			http.NotFound(w, r)
 			return
 		}
-		http.ServeContent(w, r, filename, stat.ModTime(), file.(interface {
-			Len() int64
-			io.ReadSeeker
-		}))
+		rs, ok := file.(io.ReadSeeker)
+		if !ok {
+			http.NotFound(w, r)
+			return
+		}
+		http.ServeContent(w, r, filename, stat.ModTime(), rs)
 		return
 	}