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/assets/icons/pen-line.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#ffffff" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+  <path d="M12 20h9" />
+  <path d="M16.376 3.622a1 1 0 0 1 3.002 3.002L7.368 18.635a2 2 0 0 1-.855.506l-2.872.838a.5.5 0 0 1-.62-.62l.838-2.872a2 2 0 0 1 .506-.854z" />
+</svg>