chore(release): bump version to 1.2.7 + 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 (1.2.7 entry)
- docs/release-1.2.7/TAG_DETAILS.md (release notes — 1.2.7 preamble
  on top of the full 1.2.6 + 1.2.5 content)

cc

src-tauri/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "qbz-nix"
-version = "1.2.6"
+version = "1.2.7"
 description = "Native high-fidelity client for Qobuz™ subscribers on Linux with Hi-Fi audio support (uses the Qobuz API; not certified by Qobuz)"
 authors = ["blitzkriegfc"]
 edition = "2021"