docs(FN-3408): document shell connection chrome contracts

- Add architecture notes describing shell connection chrome contracts
- Expand dashboard README with shell chrome expectations and integration details
- Add desktop README note aligning desktop shell behavior with documented contracts

Fusion-Task-Id: FN-3408

packages/desktop/src/__tests__/ipc.test.ts

@@ -131,6 +131,7 @@ describe("ipc handlers", () => {
     expect(channels.has("desktopLaunchMode:getContext")).toBe(true);
     expect(channels.has("desktopLaunchMode:setMode")).toBe(true);
     expect(channels.has("platform:get")).toBe(true);
+    expect(channels.has("shell:openConnectionManager")).toBe(true);
   });
 
   it("shell:getState returns desktop shell state", async () => {
@@ -181,6 +182,13 @@ describe("ipc handlers", () => {
     await expect(mocks.ipcHandlers.get("desktopRuntime:stopLocal")?.({})).resolves.toEqual({ source: "embedded-local", state: "running", port: 9999 });
   });
 
+  it("shell:openConnectionManager notifies renderer", async () => {
+    const { window } = await registerHandlers();
+    const result = mocks.ipcHandlers.get("shell:openConnectionManager")?.({});
+    expect(result).toBeUndefined();
+    expect(window.webContents.send).toHaveBeenCalledWith("shell:open-connection-manager");
+  });
+
   it("shell:saveProfile persists the helper-generated profile", async () => {
     await registerHandlers();
     const handler = mocks.ipcHandlers.get("shell:saveProfile");

packages/desktop/src/__tests__/preload.test.ts

@@ -56,15 +56,18 @@ describe("preload", () => {
       getDesktopLaunchMode: () => Promise<string>;
       getDesktopLaunchContext: () => Promise<unknown>;
       setDesktopLaunchMode: (mode: "choose" | "local" | "remote") => Promise<string>;
+      openConnectionManager: () => Promise<void>;
     }>("electronAPI");
 
     await api?.getDesktopLaunchMode();
     await api?.getDesktopLaunchContext();
     await api?.setDesktopLaunchMode("local");
+    await api?.openConnectionManager();
 
     expect(mocks.ipcRenderer.invoke).toHaveBeenCalledWith("desktopLaunchMode:getMode");
     expect(mocks.ipcRenderer.invoke).toHaveBeenCalledWith("desktopLaunchMode:getContext");
     expect(mocks.ipcRenderer.invoke).toHaveBeenCalledWith("desktopLaunchMode:setMode", "local");
+    expect(mocks.ipcRenderer.invoke).toHaveBeenCalledWith("shell:openConnectionManager");
   });
 
   it("fusionShell subscribes and unsubscribes state listener", async () => {