Navigator

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Currently, navigation between screens launches separate activities, each managing their own fragment stack. This leads to coupled, inconsistent back stack behavior across the app.

### Migration path

Each screen is a Fragment hosted inside `NavigatorActivity` and eventually `FileDisplayActivity` owns the single `FragmentContainerView` and `Navigator` instance. All navigation flows through `FDA`, giving the app a unified fragment back stack.

First iteration: `FileDisplayActivity` → `NavigatorActivity` → `CommunityFragment`
Second iteration: `ActivitiesActivity`