refactor: remove DXCC CW award and rename DXCC Mixed Mode to DXCC

- Removed dxcc-cw.json award definition
- Renamed "DXCC Mixed Mode" to "DXCC" in dxcc.json
- Changed award ID from "dxcc-mixed" to "dxcc"
- Removed dxcc-cw.json from awards service file list

Co-Authored-By: Claude <noreply@anthropic.com>

src/backend/services/awards.service.js

@@ -22,7 +22,6 @@ function loadAwardDefinitions() {
   try {
     const files = [
       'dxcc.json',
-      'dxcc-cw.json',
       'was.json',
       'vucc-sat.json',
       'sat-rs44.json',