fix(flutter): pubspec version must be 3-part semver

flutter pub get rejects "2026.05.11.0+1" — it requires strict
MAJOR.MINOR.PATCH+BUILD with no leading zeros. Use 2026.5.11+1
as the local default; CI's --build-name="${TAG#v}" still injects
the full 4-part tag for release builds, so production APKs report
the tag verbatim while local dev builds get a sensible recent value.

flutter_client/pubspec.yaml

@@ -1,7 +1,7 @@
 name: minstrel
 description: Minstrel mobile client
 publish_to: 'none'
-version: 2026.05.11.0+1
+version: 2026.5.11+1
 
 environment:
   sdk: '>=3.5.0 <4.0.0'