feat: inject version from release tag at build time

pubspec.yaml now holds a placeholder (0.0.0+0) that is never updated.
CI strips the v* tag to derive --build-name (e.g. 26.03.12) and
--build-number (e.g. 260312) and passes them to flutter build apk.

pubspec.yaml

@@ -2,7 +2,7 @@ name: fabled_app
 description: "FabledAssistant mobile client for Android."
 publish_to: 'none'
 
-version: 26.03.12+1
+version: 0.0.0+0  # overridden at build time via --build-name / --build-number
 
 environment:
   sdk: ^3.11.0