feat(myqbz): create-new Mixtape/Collection modal + wiring

Phase-2 Slice 4. The grid "+ New" buttons open a create modal (name + kind
radio: mixtape/collection, artist_collection excluded), create via the shared
crate, and drop the user into the new collection's detail (1:1 with Tauri's
submitCreateModal -> openMixtapeDetail). The new row prepends to its kind.

- ui/myqbz/CreateMyQbzModal.slint (mirrors CreatePlaylistModal/CreateFolderModal
  idiom; spec 30 create-modal geometry).
- MyQbzCreateState/MyQbzCreateActions in state.slint; mounted in AppShell.
- src/myqbz.rs create_collection via with_db -> with_connection ->
  qbz_mixtape::repo::create_collection (Manual, 80-char cap).
- Un-stubbed on_create_mixtape/on_create_collection; trim + busy guards; toast.

Backend crates untouched. cargo check -p qbz-slint green.

cc

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

@@ -20,6 +20,7 @@ import { PlaylistPickerModal } from "../primitives/PlaylistPickerModal.slint";
 import { CreatePlaylistModal } from "../primitives/CreatePlaylistModal.slint";
 import { EditPlaylistModal } from "../primitives/EditPlaylistModal.slint";
 import { CreateFolderModal } from "../primitives/CreateFolderModal.slint";
+import { CreateMyQbzModal } from "../myqbz/CreateMyQbzModal.slint";
 import { EphemeralPlayChoiceModal } from "../primitives/EphemeralPlayChoiceModal.slint";
 import { FolderEditModal } from "../primitives/FolderEditModal.slint";
 import { LibFolderEditModal } from "../settings/LibFolderEditModal.slint";
@@ -381,6 +382,9 @@ export component AppShell inherits Rectangle {
     // Global "Create folder" modal.
     CreateFolderModal { }
 
+    // Global "Create Mixtape / Collection" modal (My QBZ "+ New").
+    CreateMyQbzModal { }
+
     // Ephemeral "already playing" add-vs-replace choice.
     EphemeralPlayChoiceModal { }