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/mobile/src/types.ts

@@ -62,3 +62,8 @@ export interface FusionShellApi {
   openConnectionManager(): Promise<void>;
   subscribe(listener: (state: ShellConnectionState) => void): () => void;
 }
+
+export interface MobileShellDashboardBridge {
+  getState?: () => Promise<ShellConnectionState>;
+  openConnectionManager?: () => Promise<void>;
+}