feat: register Freebuff provider in all index files

- Add Freebuff to providers registry (open-sse/providers/registry/index.js)
- Register FreebuffExecutor (open-sse/executors/index.js)
- Add Freebuff OAuth config (src/lib/oauth/providers/index.js)
- Register Freebuff usage handler (open-sse/services/usage.js)

This enables Freebuff provider to appear in dashboard and be fully functional.

src/lib/oauth/providers/index.js

@@ -15,6 +15,7 @@ import qoder from "./qoder.js";
 import github from "./github.js";
 import kiro from "./kiro.js";
 import cursor from "./cursor.js";
+import freebuff from "./freebuff.js";
 import kimi from "./kimi.js";
 import kilocode from "./kilocode.js";
 import cline from "./cline.js";
@@ -40,6 +41,7 @@ const PROVIDERS = {
   github,
   kiro,
   cursor,
+  freebuff,
   kimi,
   kilocode,
   cline,