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/databaselocation.h

@@ -0,0 +1,83 @@
+/*
+    SPDX-FileCopyrightText: 2026 ToServeTheKing <[email protected]>
+
+    SPDX-License-Identifier: GPL-3.0-or-later
+*/
+
+#pragma once
+
+#include <QObject>
+#include <QQmlEngine>
+#include <QUrl>
+
+/**
+ * Where the database lives, as chosen by the user: the first-run dialog and
+ * the Preferences page drive this. The choice is stored in kareerrc
+ * ([Database] Path) and pushed into JobsDatabase::setConfiguredPath(); the
+ * --db option and KAREER_DB_PATH still win over it (see JobsDatabase::defaultPath()).
+ *
+ * Emits changed() after every switch; Main.qml reacts by refreshing the
+ * models, which reopen themselves via JobsDatabase::reopenIfPathChanged().
+ */
+class DatabaseLocation : public QObject
+{
+    Q_OBJECT
+    QML_ELEMENT
+    QML_SINGLETON
+
+    /// The effective database file, or empty while the first-run choice is pending.
+    Q_PROPERTY(QString path READ path NOTIFY changed)
+    /// $XDG_DATA_HOME/kareer/kareer.sqlite.
+    Q_PROPERTY(QString standardPath READ standardPath CONSTANT)
+    Q_PROPERTY(bool setupPending READ setupPending NOTIFY changed)
+    /// The configured file that could not be found (so setup is being asked again), if any.
+    Q_PROPERTY(QString missingPath READ missingPath NOTIFY changed)
+    /// True when --db or KAREER_DB_PATH is in effect; the stored location is then ignored.
+    Q_PROPERTY(bool overridden READ overridden CONSTANT)
+    Q_PROPERTY(QString lastError READ lastError NOTIFY messageChanged)
+    Q_PROPERTY(QString lastNotice READ lastNotice NOTIFY messageChanged)
+
+public:
+    explicit DatabaseLocation(QObject *parent = nullptr);
+
+    /// Reads kareerrc into JobsDatabase::setConfiguredPath(). Call once at
+    /// startup, for both the GUI and the CLI, so they share one database.
+    static void loadConfiguredPath();
+
+    /// True when the GUI should ask where the database lives: nothing forces
+    /// a path and the file it would open does not exist yet.
+    static bool needsSetup();
+
+    QString path() const;
+    QString standardPath() const;
+    bool setupPending() const;
+    QString missingPath() const;
+    bool overridden() const;
+    QString lastError() const;
+    QString lastNotice() const;
+
+    /// Create (or reuse) the database at the standard location.
+    Q_INVOKABLE bool useDefault();
+    /// Create (or reuse) kareer.sqlite inside the chosen folder.
+    Q_INVOKABLE bool createIn(const QUrl &folder);
+    /// Use an existing database file where it is.
+    Q_INVOKABLE bool useFile(const QUrl &file);
+    /// Copy the current database into the chosen folder and switch to the
+    /// copy. The original file is left in place.
+    Q_INVOKABLE bool moveTo(const QUrl &folder);
+
+    Q_INVOKABLE void clearMessages();
+
+Q_SIGNALS:
+    void changed();
+    void messageChanged();
+
+private:
+    /// Validates path, stores it (empty = standard location), and switches to it.
+    bool switchTo(const QString &path, bool mustExist);
+    void setMessages(const QString &error, const QString &notice);
+
+    QString m_missingPath;
+    QString m_lastError;
+    QString m_lastNotice;
+};