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.lock

@@ -4910,7 +4910,7 @@ dependencies = [
 
 [[package]]
 name = "qbz-nix"
-version = "1.2.6"
+version = "1.2.7"
 dependencies = [
  "aes-gcm",
  "anyhow",