Remove Phoenix & Notebooks - Phase 5: Remove Phoenix and the notebook container/allocation core

Deletes the Phoenix client, notebook container/allocation core, and the legacy ResourceTreeAdapter; rewires the surviving non-notebook Synapse Link state into a new minimal useSynapseLink store; removes dead Phoenix container command-bar UI; and simplifies database-shell gating to enableCloudShell. Database shells (Mongo/Cassandra/Postgres/VCoreMongo) continue to run on CloudShell.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/Explorer/Explorer.test.tsx

@@ -26,9 +26,9 @@ jest.mock("./Controls/Dialog", () => ({
   },
 }));
 
-// Silence useNotebook subscription calls
-jest.mock("./Notebook/useNotebook", () => ({
-  useNotebook: {
+// Silence useSynapseLink state calls
+jest.mock("./useSynapseLink", () => ({
+  useSynapseLink: {
     subscribe: jest.fn(),
     getState: jest.fn().mockReturnValue(
       new Proxy(