[Refactor] Migrate main screen to Jetpack Compose

Sub-issue for #368, tracking the main screen (`MainActivity`, the main entrypoint into the app)

## Tasks

* [ ] Add main screen composable (mostly done, not pushed yet)
* [x] Use Material3 Adaptive `NavigationSuiteScaffold` (done, not pushed yet)
* [x] Rewrite/remove shared FAB/BottomAppBar logic (see also #492)
* [x] Hook up `SnackbarHostState` with `MainViewModel` to the screen composable
* [ ] Use Jetpack Compose `NavHost`
* [x] Hook up dark theme/edge-to-edge/dynamic theme logic from appearance settings
* [x] Remove existing fragment transactions logic once completely migrated (done as part of #456)