refactor(runtimes): split monolithic page, add zustand store, time range selector, and delete support

- Split 1189-line runtimes-page.tsx into focused sub-components (list, detail, ping, usage, 5 chart files, shared UI, utils)
- Add useRuntimeStore zustand store for shared runtime state across pages (agents page now uses it too)
- Add 7d/30d/90d time range selector to usage charts
- Add full-stack runtime delete: SQL query, Go handler, API route, frontend with confirmation dialog
- Remove unused daemon:heartbeat WS listener (server never broadcasts it)

server/internal/handler/runtime.go

@@ -192,6 +192,26 @@ func (h *Handler) GetRuntimeTaskActivity(w http.ResponseWriter, r *http.Request)
 	writeJSON(w, http.StatusOK, resp)
 }
 
+func (h *Handler) DeleteAgentRuntime(w http.ResponseWriter, r *http.Request) {
+	runtimeID := chi.URLParam(r, "runtimeId")
+	workspaceID := resolveWorkspaceID(r)
+
+	if _, ok := h.requireWorkspaceMember(w, r, workspaceID, "runtime not found"); !ok {
+		return
+	}
+
+	err := h.Queries.DeleteAgentRuntime(r.Context(), db.DeleteAgentRuntimeParams{
+		ID:          parseUUID(runtimeID),
+		WorkspaceID: parseUUID(workspaceID),
+	})
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "failed to delete runtime")
+		return
+	}
+
+	w.WriteHeader(http.StatusNoContent)
+}
+
 func (h *Handler) ListAgentRuntimes(w http.ResponseWriter, r *http.Request) {
 	workspaceID := resolveWorkspaceID(r)
 	if _, ok := h.requireWorkspaceMember(w, r, workspaceID, "workspace not found"); !ok {