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

io.github.toservetheking.Kareer.yml

@@ -8,9 +8,12 @@ runtime-version: '6.10'
 sdk: org.kde.Sdk
 command: kareer
 
-# A job application tracker: no host filesystem, network, or portal access is
-# needed. Everything it stores lives in the app's own XDG data directory,
-# which Flatpak grants by default.
+# A job application tracker: no host filesystem or network access is needed.
+# By default everything lives in the app's own XDG data directory, which
+# Flatpak grants. A database the user places elsewhere (first-run dialog or
+# Preferences) is reached through the file chooser portal instead of a
+# --filesystem permission; folders are picked for new/moved databases so
+# SQLite can create its journal next to the file.
 finish-args:
   - --share=ipc
   - --socket=fallback-x11