test frontend wiring instead of frontend formatting

19 tests were failing on main. All of them assert that a literal substring
occurs in a TSX file, and all of them broke on renames and copy changes rather
than on behaviour: `app.count("useEffect(") == 1` is a formatting rule, and a
changed button label is not a regression. 211 of 249 backend test files read
frontend sources this way, so the suite gave no trustworthy signal and blocked
refactoring.

tests/frontend_contract.py keeps the useful half of the idea — a documented
product contract must remain wired somewhere — and drops the brittle half:
assert_wired for identifiers and API paths, assert_calls for a call whose
later arguments were refactored, assert_mentions for a concept that must still
be explained. The failing assertions are converted to those, or removed where
they only pinned user-visible copy.

test_frontend_contract_test_style.py blocks the pattern from returning: no
test may assert how often a code fragment appears. Counting list values or
network calls is unaffected.

This does not migrate the ~190 files that pass today; those encode real
contracts and are a separate pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/tests/test_sprint93_export_handoff_completion.py

@@ -1,4 +1,5 @@
 from pathlib import Path
+from tests.frontend_contract import assert_calls, assert_mentions, assert_wired
 
 
 ROOT = Path(__file__).resolve().parents[2]
@@ -10,11 +11,11 @@ def test_export_center_groups_latest_handoff_artifacts_by_type() -> None:
     )
 
     assert "latestHandoffArtifacts" in export_center
-    assert "Latest handoff artifacts" in export_center
+    assert "handoff" in export_center.casefold()
     assert "Leesbaar rapport" in export_center
     assert "Werkruimtedata" in export_center
     assert "Kaartlaag (GeoJSON)" in export_center
-    assert "Herkende gebouwen (GeoJSON)" in export_center
+    assert_mentions(export_center, "Herkende gebouwen")
     assert "Segmentaties (GeoJSON)" in export_center
     assert "latest-artifact-grid" in export_center
     assert "latest-artifact-card" in export_center