Add --db and let the user choose where the database lives

- `kareer --db <path>` works for the GUI and every subcommand, before
  or after the subcommand name, and creates the file if missing. main()
  strips it from argv before CLI/GUI routing; every parser declares it
  so it shows in --help.
- Path precedence: --db > KAREER_DB_PATH > the location chosen in the
  GUI (kareerrc [Database] Path, read at startup for GUI and CLI alike)
  > $XDG_DATA_HOME/kareer/kareer.sqlite.
- First GUI run (nothing forced, no file yet, or the configured file
  has gone missing): DatabaseSetupDialog offers the default location, a
  folder of the user's choice, or an existing database used in place.
  Until then JobsDatabase opens nothing.
- Preferences gains a Database section: Move to... (copies, leaves the
  original), Open Existing..., Use Default Location. Refuses foreign
  SQLite files and unwritable ones.
- Models call JobsDatabase::reopenIfPathChanged() on refresh, so
  switching databases needs no restart.
- Link KF6::ConfigCore; add a QuickDialogs2 configure-time guard; add
  kconfig to the Arch dependencies and a note to the Flatpak manifest.
- README documents --db, the precedence, and the first-run choice, and
  gains Fedora 44 build dependencies; CONTRIBUTING gets a dev recipe.

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

src/qml/Main.qml

@@ -25,6 +25,27 @@ Kirigami.ApplicationWindow {
         id: jobsModel
     }
 
+    // The database moved (first-run choice or Preferences): reload the list,
+    // which cascades to the dashboard via countChanged, and leave any open
+    // edit form, since its job id belonged to the previous database.
+    Connections {
+        target: DatabaseLocation
+        function onChanged(): void {
+            jobsModel.refresh();
+            root.showDashboard();
+        }
+    }
+
+    DatabaseSetupDialog {
+        id: setupDialog
+    }
+
+    Component.onCompleted: {
+        if (DatabaseLocation.setupPending) {
+            setupDialog.open();
+        }
+    }
+
     // The job currently open in the edit form, so the sidebar can keep it highlighted.
     property int currentJobId: -1