hotfix(cliproxy): fallback from deleted CLIProxyAPIPlus to original backend

The upstream router-for-me/CLIProxyAPIPlus repo was deleted, breaking any
install/update path that resolved backend: plus. Existing users with
`backend: plus` saved in config.yaml hit 404s on every bootstrap.

Changes:
- Flip DEFAULT_BACKEND from 'plus' to 'original' (platform-detector.ts)
- Flip default cliproxy.backend in createEmptyUnifiedConfig() to 'original'
- Runtime 404 fallback: getConfiguredBackend() degrades 'plus' -> 'original'
  with a one-time warning, so existing installations keep working without a
  manual reconfig step
- Update CLIProxyBackend docblock to document the fallback behavior
- Update tests to match new default

Retains the 'plus' type and BACKEND_CONFIG entry for forward compatibility
once CCS self-maintains its own Plus fork.

Closes #1062

tests/unit/cliproxy/binary-manager-install.test.ts

@@ -78,7 +78,7 @@ describe('installCliproxyVersion', () => {
         skipAutoUpdate: true,
       })
     ).rejects.toThrow(
-      'CLIProxy Plus binary is not installed locally. Run "ccs cliproxy install" when you have network access.'
+      'CLIProxy binary is not installed locally. Run "ccs cliproxy install" when you have network access.'
     );
   });
 });