chore(repo): pin Flutter to the melos min via .fvmrc

Adds .fvmrc with the same Flutter floor the workspace declares in
melos.yaml (>=3.27.4) so contributors running fvm pick up the
right SDK automatically.

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

Author: xsahil03x

.fvmrc

@@ -0,0 +1,3 @@
+{
+  "flutter": "3.27.4"
+}