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

CMakeLists.txt

@@ -36,12 +36,16 @@ find_package(Qt6 ${REQUIRED_QT_VERSION} REQUIRED COMPONENTS
     # QtQuick.Shapes runtime QML module (SankeyDiagram.qml); configure-time
     # guard only, nothing links against it.
     QuickShapesPrivate
+    # QtQuick.Dialogs runtime QML module (DatabaseSetupDialog.qml,
+    # SettingsPage.qml); configure-time guard only, nothing links against it.
+    QuickDialogs2
     QuickControls2
     Sql
     Test
 )
 
 find_package(KF6 ${REQUIRED_KF_VERSION} REQUIRED COMPONENTS
+    Config
     I18n
     CoreAddons
     IconThemes