fix: take the file edit limit in the panel from the agent's read cap

The limit was repeated in the page; a file of exactly the cap is read
in full, so it is editable too.

internal/agent/fs.go

@@ -17,8 +17,8 @@ import (
 	"github.com/x-grk/longpolling-remote-access/internal/proto"
 )
 
-// fsReadCap bounds how many bytes a view/read returns (5 MiB).
-const fsReadCap = 5 * 1024 * 1024
+// fsReadCap bounds how many bytes a view/read returns.
+const fsReadCap = proto.FSReadCap
 
 // fsDownloadCap bounds a browser download (kept under the 64 MiB result/body
 // limit once base64-encoded).