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/qml/SettingsPage.qml

@@ -11,6 +11,7 @@ pragma ComponentBehavior: Bound
 // header, no card/border around them.
 import QtQuick
 import QtQuick.Controls as QQC2
+import QtQuick.Dialogs
 import QtQuick.Layouts
 import org.kde.kirigami as Kirigami
 import io.github.toservetheking.Kareer
@@ -75,5 +76,94 @@ Kirigami.ScrollablePage {
                 displayText: AppColorScheme.activeColorSchemeName
             }
         }
+
+        Kirigami.ListSectionHeader {
+            Layout.fillWidth: true
+            text: i18nc("@title:group", "Database")
+        }
+
+        ColumnLayout {
+            Layout.fillWidth: true
+            Layout.leftMargin: Kirigami.Units.largeSpacing
+            Layout.rightMargin: Kirigami.Units.largeSpacing
+            Layout.topMargin: Kirigami.Units.smallSpacing
+            Layout.bottomMargin: Kirigami.Units.smallSpacing
+            spacing: Kirigami.Units.smallSpacing
+
+            QQC2.Label {
+                Layout.fillWidth: true
+                text: i18nc("@label", "Location")
+            }
+            QQC2.Label {
+                Layout.fillWidth: true
+                text: DatabaseLocation.path
+                wrapMode: Text.WrapAnywhere
+                opacity: 0.7
+            }
+
+            Kirigami.InlineMessage {
+                Layout.fillWidth: true
+                type: Kirigami.MessageType.Information
+                text: i18nc("@info", "Set by the --db option or the KAREER_DB_PATH environment variable, so it can't be changed here.")
+                visible: DatabaseLocation.overridden
+            }
+            Kirigami.InlineMessage {
+                Layout.fillWidth: true
+                type: Kirigami.MessageType.Error
+                text: DatabaseLocation.lastError
+                visible: text.length > 0
+            }
+            Kirigami.InlineMessage {
+                Layout.fillWidth: true
+                type: Kirigami.MessageType.Positive
+                text: DatabaseLocation.lastNotice
+                visible: text.length > 0
+            }
+
+            Flow {
+                Layout.fillWidth: true
+                spacing: Kirigami.Units.smallSpacing
+                enabled: !DatabaseLocation.overridden
+
+                QQC2.Button {
+                    icon.name: "folder-move"
+                    text: i18nc("@action:button", "Move to…")
+                    QQC2.ToolTip.text: i18nc("@info:tooltip", "Copy the database into another folder and use the copy from now on. The current file is left in place.")
+                    QQC2.ToolTip.visible: hovered
+                    QQC2.ToolTip.delay: Kirigami.Units.toolTipDelay
+                    onClicked: moveFolderDialog.open()
+                }
+                QQC2.Button {
+                    icon.name: "document-open"
+                    text: i18nc("@action:button", "Open Existing…")
+                    onClicked: openFileDialog.open()
+                }
+                QQC2.Button {
+                    icon.name: "edit-reset"
+                    text: i18nc("@action:button", "Use Default Location")
+                    visible: DatabaseLocation.path !== DatabaseLocation.standardPath
+                    QQC2.ToolTip.text: xi18nc("@info:tooltip", "Switch to <filename>%1</filename>. Nothing is copied.", DatabaseLocation.standardPath)
+                    QQC2.ToolTip.visible: hovered
+                    QQC2.ToolTip.delay: Kirigami.Units.toolTipDelay
+                    onClicked: DatabaseLocation.useDefault()
+                }
+            }
+        }
+    }
+
+    Component.onCompleted: DatabaseLocation.clearMessages()
+
+    FolderDialog {
+        id: moveFolderDialog
+        title: i18nc("@title:window", "Move Database To")
+        onAccepted: DatabaseLocation.moveTo(selectedFolder)
+    }
+
+    FileDialog {
+        id: openFileDialog
+        title: i18nc("@title:window", "Open Database")
+        fileMode: FileDialog.OpenFile
+        nameFilters: [i18nc("@item:inlistbox", "Kareer databases (*.sqlite)"), i18nc("@item:inlistbox", "All files (*)")]
+        onAccepted: DatabaseLocation.useFile(selectedFile)
     }
 }