feat: right-click context menus for channels, groups, and servers

ContextMenu: reusable hook-based component that positions a menu at
the cursor and auto-closes on outside click or Escape.

ChannelList:
- Right-click channel -> Edit Name, Permissions, Delete Channel
- Right-click group header -> Edit Name, Create Channel Here, Delete Group
- Inline rename: replacing name text with an input, commits on Enter/blur
- CreateChannelModal now accepts defaultGroupId to pre-select a group

ServerBar:
- Right-click server icon -> Server Settings, Invite People, Leave Server
- Leave calls DELETE /servers/{id}/members/me

Backend:
- Added LeaveServer handler (DELETE /servers/{serverID}/members/me)
- Server owner cannot leave; must transfer or delete

internal/server/handlers.go

@@ -24,6 +24,7 @@ func (h *Handler) RegisterRoutes(r chi.Router) {
 	r.Get("/{serverID}", h.Get)
 	r.Patch("/{serverID}", h.Update)
 	r.Delete("/{serverID}", h.Delete)
+	r.Delete("/{serverID}/members/me", h.LeaveServer)
 }
 
 type createServerRequest struct {
@@ -315,3 +316,38 @@ func (h *Handler) Delete(w http.ResponseWriter, r *http.Request) {
 
 	w.WriteHeader(http.StatusNoContent)
 }
+
+func (h *Handler) LeaveServer(w http.ResponseWriter, r *http.Request) {
+	userID, ok := middleware.UserIDFromContext(r.Context())
+	if !ok {
+		http.Error(w, `{"error":"unauthorized"}`, http.StatusUnauthorized)
+		return
+	}
+
+	serverID := chi.URLParam(r, "serverID")
+
+	// Cannot leave a server you own
+	var ownerID string
+	err := h.db.QueryRowContext(r.Context(), `SELECT owner_id FROM servers WHERE id = $1`, serverID).Scan(&ownerID)
+	if err != nil {
+		http.Error(w, `{"error":"server not found"}`, http.StatusNotFound)
+		return
+	}
+	if ownerID == userID {
+		http.Error(w, `{"error":"server owner cannot leave; transfer ownership or delete the server"}`, http.StatusForbidden)
+		return
+	}
+
+	result, err := h.db.ExecContext(r.Context(), `DELETE FROM members WHERE user_id = $1 AND server_id = $2`, userID, serverID)
+	if err != nil {
+		http.Error(w, `{"error":"failed to leave server"}`, http.StatusInternalServerError)
+		return
+	}
+	rows, _ := result.RowsAffected()
+	if rows == 0 {
+		http.Error(w, `{"error":"not a member"}`, http.StatusNotFound)
+		return
+	}
+
+	w.WriteHeader(http.StatusNoContent)
+}