fix(header): wire the Documentation menu item to the project Wiki

The hamburger "Documentation" entry had no clicked handler, so it did
nothing. Point it at github.com/vicrodh/qbz/wiki via the generic
external-link handler.

cc

crates/qbz-ui/ui/shell/HeaderBar.slint

@@ -14,7 +14,7 @@ import { Spacing } from "../foundation/spacing.slint";
 import { Typography } from "../foundation/typography.slint";
 import { Layout } from "../foundation/layout.slint";
 import { Radius } from "../foundation/radius.slint";
-import { SessionState, NavState, ContentView, ShellState, HeaderMenuState, NavMenuEntry, SearchActions, SearchState, SidebarPlaylistsPopupState, OfflineState, SettingsState, OfflineModeActions, AppearanceState, KeyboardShortcutsState, LinkResolverState, UiFocusState, ReportIssueState, AboutState, WhatsNewActions } from "../state.slint";
+import { SessionState, NavState, ContentView, ShellState, HeaderMenuState, NavMenuEntry, SearchActions, SearchState, SidebarPlaylistsPopupState, OfflineState, SettingsState, OfflineModeActions, AppearanceState, KeyboardShortcutsState, LinkResolverState, UiFocusState, ReportIssueState, AboutState, WhatsNewActions, AlbumActions } from "../state.slint";
 import { QbzIcon } from "../primitives/QbzIcon.slint";
 import { SecondaryButton } from "../primitives/SecondaryButton.slint";
 import { QbzToggle } from "../primitives/QbzToggle.slint";
@@ -1097,6 +1097,12 @@ export component HeaderBar inherits Rectangle {
                 MenuItem {
                     icon: @image-url("../assets/icons/book-open.svg");
                     label: @tr("Documentation");
+                    clicked => {
+                        menu.close();
+                        // Opens in the system browser (generic external-link
+                        // handler). Points at the project Wiki.
+                        AlbumActions.open-external-link("https://github.com/vicrodh/qbz/wiki");
+                    }
                 }
                 MenuItem {
                     icon: @image-url("../assets/icons/bug.svg");