Fix the edit form so Save persists stage changes

Changing an application's stage and pressing Save closed the form
without saving the stage: JobsDatabase::updateJob() deliberately skips
the stage column so every move lands in stage_history, but
JobEditModel::save() never followed up with setStage(). It now does,
as the CLI's update command always has.

Also:
- Validate that company and title are non-empty and say so, instead of
  saving a blank field
- Always set lastError on failure, clear it on the next attempt, and
  bind the page's error message to it
- Reload the form's values when jobsModel is assigned, since QML does
  not guarantee it is set before editingJobId
- Bind the salary spin boxes to the model and write back only on user
  edits; guard the notes field the same way
- Add jobeditmodeltest covering load order, edits, stage history,
  salaries round-tripping, adding, and validation errors

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01BxDf7HqD1xPnsZP8wt3NTk

autotests/CMakeLists.txt

@@ -39,3 +39,25 @@ target_link_libraries(sankeylayouttest PRIVATE
 )
 
 add_test(NAME sankeylayouttest COMMAND sankeylayouttest)
+
+add_executable(jobeditmodeltest
+    jobeditmodeltest.cpp
+    ../src/jobstage.cpp
+    ../src/jobsdatabase.cpp
+    ../src/jobsmodel.cpp
+    ../src/jobeditmodel.cpp
+    ../src/jobfieldcatalog.cpp
+)
+
+target_include_directories(jobeditmodeltest PRIVATE ../src)
+
+target_link_libraries(jobeditmodeltest PRIVATE
+    Qt6::Core
+    Qt6::Gui
+    Qt6::Qml
+    Qt6::Sql
+    Qt6::Test
+    KF6::I18n
+)
+
+add_test(NAME jobeditmodeltest COMMAND jobeditmodeltest)