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/preload.ts

@@ -59,6 +59,7 @@ const electronApi = {
     ipcRenderer.invoke("desktopLaunchMode:setMode", mode),
   getDesktopLaunchContext: (): Promise<{ mode: "remote"; profileId: string; serverBaseUrl: string; serverLabel?: string; authToken?: string } | null> =>
     ipcRenderer.invoke("desktopLaunchMode:getContext"),
+  openConnectionManager: (): Promise<void> => ipcRenderer.invoke("shell:openConnectionManager"),
 
   // Tray status
   updateTrayStatus: (status: string): Promise<void> => ipcRenderer.invoke("tray:updateStatus", status),