refactor(qconnect): move device_uuid persistence to qbz-app (slice 1)

Relocate the persistent QConnect device-identity resolution out of the
Tauri adapter into the frontend-agnostic qbz-app crate
(qbz-app/src/qconnect_identity.rs) so both the Tauri and a future Slint
frontend resolve the SAME uuid for the same install.

Moved: resolve_qconnect_device_uuid, device_uuid_from_db,
load/persist_device_uuid, qconnect_settings_db_path.

Byte-identical persistence contract preserved: DB path
(<data_dir>/qbz/qconnect_settings.db), the 'device_uuid' key, and the
QBZ_QCONNECT_DEVICE_UUID env override are unchanged, so existing installs
keep their uuid. The Tauri transport.rs::resolve_qconnect_device_uuid is
now a one-line delegate; the device-NAME persistence shares the same DB
file via the qbz-app path helper.

qbz-app gains a uuid dependency (it already had rusqlite + dirs, so no
SQLite dep was added to qconnect-app per R6). The 2 device_uuid tests
move to qbz-app.

cc

src-tauri/Cargo.lock

@@ -5039,6 +5039,7 @@ dependencies = [
  "serde",
  "serde_json",
  "tokio",
+ "uuid 1.23.1",
 ]
 
 [[package]]