Split UI test monolith into focused TUs

test_encryption_app.cpp was 1466 lines. Extract the three heaviest
test methods + one helper into their own translation units. All
methods remain on TestOpenCryptUI (declared in test_encryption_app.h)
so moc and test discovery continue to work unchanged.

tests/test_ui_encrypt_decrypt.cpp (new, 285 lines)
  - testEncryptDecrypt
  - encryptAndDecrypt helper
  UI-driven single-file round-trip.

tests/test_ui_smoke.cpp (new, 194 lines)
  - testTabSwitching
  - testCryptoProviderSwitching

tests/test_ui_tamper.cpp (new, 89 lines)
  - testTamperDetection
  Engine-level coverage lives in test_engine_tamper.cpp;
  this drives the same flow via real widgets.

tests/test_ui_common.h (new, 30 lines)
  Shared Qt + engine include block so each test_ui_*.cpp file
  starts with a single #include instead of repeating 20 lines.

tests/test_encryption_app.cpp: 1466 → 914 lines. Still the largest
test TU but now sits below the 1000-line threshold and contains
only init/cleanup/helpers + the small QSKIP'd / conditionally-
skipped tests.

Verified: ctest -j 4 reports 100% (9/9 in 41 s).

No files in src/, include/, or tests/ exceed 1000 lines now:
  914 tests/test_encryption_app.cpp
  837 src/mainwindow_disk.cpp
  800 src/encryptionengine_keyderivation.cpp
  705 src/encryptionengine_crypto.cpp
  668 src/mainwindow.cpp

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bram

CMakeLists.txt

@@ -199,6 +199,10 @@ include_directories(${CMAKE_CURRENT_BINARY_DIR})
 set(TEST_SOURCES
     tests/main.cpp
     tests/test_encryption_app.cpp
+    tests/test_ui_encrypt_decrypt.cpp
+    tests/test_ui_smoke.cpp
+    tests/test_ui_tamper.cpp
+    tests/test_ui_common.h
 )
 
 # Create test executable