chore(release): bump to 1.2.15

Version bump across Cargo/tauri/npm/snap/flake, metainfo release entry,
and TAG_DETAILS for 1.2.15. flake.nix hashes cleared (filled post-tag
via nix build once v1.2.15 exists).

cc

package.json

@@ -1,6 +1,6 @@
 {
   "name": "qbz-nix",
-  "version": "1.2.14",
+  "version": "1.2.15",
   "description": "Native high-fidelity client for Qobuz™ subscribers on Linux with Hi-Fi audio support (uses the Qobuz API; not certified by Qobuz)",
   "type": "module",
   "scripts": {