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/CMakeLists.txt

@@ -21,6 +21,7 @@ qt_add_qml_module(kareer
         qml/DashboardPage.qml
         qml/SankeyDiagram.qml
         qml/SettingsPage.qml
+        qml/DatabaseSetupDialog.qml
     SOURCES
         jobsmodel.cpp
         jobsmodel.h
@@ -32,6 +33,8 @@ qt_add_qml_module(kareer
         jobeditmodel.h
         appcolorscheme.cpp
         appcolorscheme.h
+        databaselocation.cpp
+        databaselocation.h
 )
 
 target_include_directories(kareer PRIVATE ${CMAKE_BINARY_DIR})
@@ -44,6 +47,7 @@ target_link_libraries(kareer PRIVATE
     Qt6::Quick
     Qt6::QuickControls2
     Qt6::Sql
+    KF6::ConfigCore
     KF6::I18n
     KF6::I18nQml
     KF6::CoreAddons