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

src/jobeditmodel.h

@@ -74,10 +74,12 @@ Q_SIGNALS:
 
 private:
     void resetValues();
+    void setLastError(const QString &error);
 
     JobsModel *m_jobsModel = nullptr;
     int m_editingJobId = -1;
     QHash<QString, QVariant> m_values;
     QString m_lastError;
+    QString m_loadedStage; ///< Stage as loaded from the database, to detect stage changes on save.
     QList<JobFieldCatalog::Field> m_fields;
 };