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

CONTRIBUTING.md

@@ -16,6 +16,22 @@ Every GUI action is also available from the `kareer` CLI
 (`add|list|show|update|stage|delete|stats|stages`) — run `./build/bin/kareer
 --help` to see the subcommands.
 
+To develop against a throwaway database instead of your real one, pass
+`--db` (the file is created if it doesn't exist); it works for the GUI and
+every subcommand:
+
+```sh
+./build/bin/kareer --db build/dev.sqlite add --company Acme --title Engineer
+./build/bin/kareer --db build/dev.sqlite
+```
+
+To see the first-run "where should the database live?" dialog, start with
+empty config and data directories:
+
+```sh
+XDG_CONFIG_HOME=$(mktemp -d) XDG_DATA_HOME=$(mktemp -d) ./build/bin/kareer
+```
+
 ## Test
 
 ```sh