fold redundant test_coverage.sh into the gate

`ci/test.sh` now runs `flutter test --coverage`, so
`ci/test_coverage.sh` was just re-running the same tests plus an
optional `lcov --summary` that needs `lcov` installed (it wasn't,
on at least this machine). Removing it.

- ci/test_coverage.sh: deleted.
- Makefile: drop the `coverage` target (it only wrapped the dead
  script). Fix a stale `coverage/floor.txt` reference in the
  `coverage-gate` help text — the floor lives in pubspec.yaml now.
- .gitea/workflows/test.yml: replace the test_coverage.sh invocation
  with ci/coverage_gate.sh, so CI enforces the same floor as the
  pre-push hook (defense in depth).

Co-Authored-By: Claude <noreply@anthropic.com>

.gitea/workflows/test.yml

@@ -26,7 +26,7 @@ jobs:
       - run: (cd app && flutter pub get)
       - run: ci/test.sh
       - run: ci/test_a11y.sh
-      - run: ci/test_coverage.sh
+      - run: ci/coverage_gate.sh
 
   integration:
     name: integration_test (xvfb)