app: deep-freeze App state to prevent accidental mutation

In `packages/modules/app/src/app/App.ts`, there's a `// todo deep-freeze` comment marking a spot where the app state should be deep-frozen to prevent consumers from accidentally mutating shared state objects.

Read packages/modules/app/src/app/App.ts and locate the `// todo deep-freeze` comment. Trace how the app state is assembled there and identify the existing way state is exposed to consumers. Done means shared app state objects are deeply frozen so accidental consumer mutation is prevented.

Written by the indexing model from the issue text.