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

internal/server/handlers.go

@@ -102,6 +102,9 @@ func (h *Handler) Create(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Seed the default @everyone role (non-fatal if it fails)
+	_ = SeedDefaultRole(r.Context(), h.db, srv.ID)
+
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(http.StatusCreated)
 	json.NewEncoder(w).Encode(srv)