ci: use --set-exit-if-changed for formatting gate (match release.yml)

Previously ci.yml used 'dart format --fix' which silently reformatste
code without failing the build. Changed to '--set-exit-if-changed' to
match release.yml — a proper gate that catches formatting violations
early in CI rather than silently fixing them.

.gitea/workflows/ci.yml

@@ -44,7 +44,7 @@ jobs:
         run: flutter analyze --fatal-warnings
 
       - name: Check formatting
-        run: dart format --fix lib/ test/
+        run: dart format --set-exit-if-changed lib/ test/
 
 
   # ── Stage 2: Unit & widget tests ─────────────────────────────────