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

snapcraft.yaml

@@ -3,7 +3,7 @@
 
 name: qbz-player
 title: "QBZ — Native Qobuz Client for Linux"
-version: '1.2.6'
+version: '1.2.7'
 summary: Native Qobuz player for Linux with bit-perfect audio and DAC control
 description: |
   QBZ is a native Linux desktop client for Qobuz™, designed for high-fidelity playback.