Fix straightforward analyzer warnings

[martin-77]

Summary

This PR cleans up a small set of straightforward Dart analyzer warnings without intentionally changing behavior.

Changes include:

• removing unused and duplicate imports
• removing unused locals / dead statements
• simplifying obviously redundant checks where safe
• keeping behavior-sensitive warnings out of scope

Validation

• flutter analyze: completes with no errors; remaining findings are pre-existing warnings/infos outside this cleanup
• dart format --output=none --set-exit-if-changed on the 14 changed Dart files: 0 changed
• repository currently has no test/ directory, so flutter test is not applicable

This is intentionally kept small and mechanical to make review straightforward.