Fix Freebuff: correct User-Agent, metadata fields, session body, preset aliases

- User-Agent: ai-sdk/openai-compatible/1.0.25/codebuff (matches official SDK)
- metadata: freebuff_instance_id (not codebuff_instance_id) + client_id field
- Session POST: empty body {} (not {model: ...})
- Add Freebuff/FreeBuff preset aliases for backward compat

src/codex-launcher-gui

@@ -400,6 +400,24 @@ PROVIDER_PRESETS = {
             "moonshotai/kimi-k2.6", "minimax/minimax-m2.7",
         ],
     },
+    "Freebuff (Free DeepSeek/Kimi)": {
+        "backend_type": "codebuff",
+        "base_url": "https://www.codebuff.com",
+        "oauth_provider": "codebuff",
+        "models": [
+            "deepseek/deepseek-v4-pro", "deepseek/deepseek-v4-flash",
+            "moonshotai/kimi-k2.6", "minimax/minimax-m2.7",
+        ],
+    },
+    "FreeBuff": {
+        "backend_type": "codebuff",
+        "base_url": "https://www.codebuff.com",
+        "oauth_provider": "codebuff",
+        "models": [
+            "deepseek/deepseek-v4-pro", "deepseek/deepseek-v4-flash",
+            "moonshotai/kimi-k2.6", "minimax/minimax-m2.7",
+        ],
+    },
 }
 
 def safe_name(name):