Wire up CTest and add unit test coverage for ProfileRepository

Adds a Qt6::Test-based unit test target (tests/test_profile_repository.cpp,
21 cases), gated behind an ORBITHUB_BUILD_TESTS option that no-ops
gracefully if Qt6::Test isn't available, so it can't break app-only
builds. Covers profile CRUD, validation rules, search/sort, tag
normalization, and folder handling, each against an isolated temporary
SQLite file (new ProfileRepository(databasePathOverride) constructor
overload added for exactly this).

Caught and fixed a real bug along the way: normalizedTags()'s result was
bound directly without the nonNullTrimmed() null-guard every other field
already uses, so creating a profile with no tags at all hit the `tags
NOT NULL` constraint and silently failed -- including via the Import
Profiles feature for any export where a profile has no tags key.

Partial progress on #1 (RdpSessionBackend/SshSessionBackend state-machine
coverage still open -- much larger lift, needs a testability pass on
those backends first).

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

CMakeLists.txt

@@ -16,6 +16,17 @@ find_package(Qt6 6.2 REQUIRED COMPONENTS Widgets Sql)
 
 qt_standard_project_setup()
 
+option(ORBITHUB_BUILD_TESTS "Build unit tests (requires Qt6::Test)" ON)
+if(ORBITHUB_BUILD_TESTS)
+    find_package(Qt6 6.2 QUIET COMPONENTS Test)
+    if(TARGET Qt6::Test)
+        enable_testing()
+    else()
+        message(STATUS "Qt6::Test not found -- skipping unit tests (set ORBITHUB_BUILD_TESTS=OFF to silence this)")
+        set(ORBITHUB_BUILD_TESTS OFF)
+    endif()
+endif()
+
 add_subdirectory(third_party/KodoTerm)
 
 if(NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/third_party/FreeRDP/CMakeLists.txt")
@@ -272,3 +283,7 @@ install(FILES third_party/KodoTerm/LICENSE
     DESTINATION ${CMAKE_INSTALL_DATADIR}/licenses/org.darksingularity.OrbitHub
     RENAME LICENSE-KodoTerm
 )
+
+if(ORBITHUB_BUILD_TESTS)
+    add_subdirectory(tests)
+endif()