Switch flet dependency to Git source

Changed the flet package source from a versioned release to a Git repository reference, pointing to the main branch of the official flet-dev/flet repository. This allows for using the latest code from the repository.

Author: FeodorFitsner

src/serious_python/example/flet_example/pubspec.yaml

@@ -35,7 +35,11 @@ dependencies:
   serious_python:
     path: ../../
 
-  flet: ^0.26.0
+  flet:
+    git:
+      path: packages/flet
+      ref: main
+      url: https://github.com/flet-dev/flet.git
 
   path: ^1.8.3
   url_strategy: ^0.2.0