refactor: remove voting system, add author colors + pin numbering

- Remove votes backend (route, db functions, server mount)
- Remove votes frontend (store, socket, PinOverlay badges, FeedbackPanel UI)
- Add authorColors utility (8-color palette, deterministic hash)
- Add getPinNumber() to annotationStore
- Fix JWT to include username/display_name
- Add resolveAuthorName DB fallback for old tokens

backend/auth.js

@@ -14,7 +14,7 @@ const REFBOARD_API_KEY = process.env.REFBOARD_API_KEY || '';
 
 function generateToken(user) {
   return jwt.sign(
-    { id: user.id, email: user.email, role: user.role },
+    { id: user.id, email: user.email, role: user.role, username: user.username, display_name: user.display_name },
     JWT_SECRET,
     { expiresIn: JWT_EXPIRES_IN }
   );