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

src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
   "productName": "QBZ",
-  "version": "1.2.5",
+  "version": "1.2.6",
   "identifier": "com.blitzfc.qbz",
   "build": {
     "beforeDevCommand": "npm run dev",