chore(release): bump version to 1.2.6 + release notes

Bumped:
- src-tauri/Cargo.toml + Cargo.lock
- src-tauri/tauri.conf.json
- package.json + package-lock.json
- snapcraft.yaml
- flake.nix (qbzVersion, hashes cleared for nix users to regenerate)

Added:
- packaging/flatpak/com.blitzfc.qbz.metainfo.xml (1.2.6 entry)
- docs/release-1.2.6/TAG_DETAILS.md (release notes)

cc

package.json

@@ -1,6 +1,6 @@
 {
   "name": "qbz-nix",
-  "version": "1.2.5",
+  "version": "1.2.6",
   "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": {