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/backend-selection.test.js

@@ -28,8 +28,8 @@ describe('Backend Selection', () => {
   });
 
   describe('DEFAULT_BACKEND', () => {
-    it('defaults to plus backend for backward compatibility', () => {
-      assert.strictEqual(platformDetector.DEFAULT_BACKEND, 'plus');
+    it('defaults to original backend (Plus upstream deleted, issue #1062)', () => {
+      assert.strictEqual(platformDetector.DEFAULT_BACKEND, 'original');
     });
   });
 
@@ -45,9 +45,10 @@ describe('Backend Selection', () => {
       assert(info.binaryName.startsWith('CLIProxyAPIPlus_6.6.51-0_'));
     });
 
-    it('uses plus backend by default', () => {
+    it('uses original backend by default (Plus upstream deleted, issue #1062)', () => {
       const info = platformDetector.detectPlatform();
-      assert(info.binaryName.includes('CLIProxyAPIPlus'));
+      assert(info.binaryName.startsWith('CLIProxyAPI_'));
+      assert(!info.binaryName.includes('CLIProxyAPIPlus'));
     });
 
     it('uses fallback version when version not specified', () => {
@@ -76,9 +77,10 @@ describe('Backend Selection', () => {
       assert.strictEqual(name, expected);
     });
 
-    it('defaults to plus backend', () => {
+    it('defaults to original backend (Plus upstream deleted, issue #1062)', () => {
       const name = platformDetector.getExecutableName();
-      assert(name.includes('cli-proxy-api-plus'));
+      const expected = isWindows ? 'cli-proxy-api.exe' : 'cli-proxy-api';
+      assert.strictEqual(name, expected);
     });
   });
 
@@ -94,9 +96,10 @@ describe('Backend Selection', () => {
       assert(url.includes('router-for-me/CLIProxyAPIPlus/releases'));
     });
 
-    it('defaults to plus backend', () => {
+    it('defaults to original backend (Plus upstream deleted, issue #1062)', () => {
       const url = platformDetector.getDownloadUrl();
-      assert(url.includes('CLIProxyAPIPlus'));
+      assert(url.includes('router-for-me/CLIProxyAPI/releases'));
+      assert(!url.includes('CLIProxyAPIPlus'));
     });
   });
 

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.'
     );
   });
 });