feat(slint): rename + delete playlist (edit modal)

Completes playlist CRUD. The detail header shows an edit button only
for the user's own playlists (is-owner = the open id is in the sidebar
list); it opens an EditPlaylistModal:

  - Save -> update_playlist (rename); refreshes the detail header name
    and the sidebar list.
  - Delete -> delete_playlist; closes the modal, navigates back, and
    refreshes the sidebar.

Adds the pen-line icon (Lucide) used by the edit button.

cc

crates/qbz-slint/ui/state.slint

@@ -243,10 +243,26 @@ export global PlaylistState {
     in property <string> cover-url;
     in property <image> cover;
     in property <bool> is-favorite: false;
+    // True when the playlist is one of the user's own (editable).
+    in property <bool> is-owner: false;
     in property <[TrackItem]> tracks: [];
     in property <bool> loading: false;
 }
 
+// "Edit playlist" modal state (rename + delete).
+export global EditPlaylistState {
+    in-out property <bool> open: false;
+    in property <string> id;
+    in-out property <string> name;
+    in property <bool> busy: false;
+}
+
+export global EditPlaylistActions {
+    callback save();
+    callback delete();
+    callback close();
+}
+
 // Settings screen state. Audio and Playback preferences are loaded from
 // the persistence stores by the Rust settings controller and pushed here;
 // changes flow back through the settings-bool / settings-select /