feat: UI for server/channel creation, settings link, role management backend

Frontend:
- Settings link in header next to logout
- [+] button to create server (name + optional icon)
- [#] button to join server via invite code
- [+] button to create channel (name, type, category)
- Server name shown instead of UUID in channel list header
- /invites/:code route wired for JoinServer component

Backend:
- Wire role CRUD + member-role assignment routes (MANAGE_SERVER gated)
- Seed @everyone default role on server creation

cmd/server/main.go

@@ -141,6 +141,10 @@ func main() {
 				})
 			})
 
+			// Roles and member-role assignment
+			roleHandler := server.NewRoleHandler(database.DB)
+			roleHandler.RegisterRoleRoutes(r)
+
 			// Messages (under channels)
 			r.Route("/channels/{channelID}/messages", func(r chi.Router) {
 				message.NewHandler(database.DB, hub, pushHandler, logger).RegisterRoutes(r)