Split AppModule.kt DI configuration by feature (269 lines)

## Problem

`AppModule.kt` is 269 lines containing all Koin DI definitions for the entire app. As the app grows, this becomes harder to navigate and maintain.

## Location

- `app/src/main/java/org/greenstand/android/TreeTracker/di/AppModule.kt`

## Suggested Fix

Split into feature-specific modules:
- `ViewModelModule` — all ViewModel definitions
- `UseCaseModule` — all UseCase definitions
- `RepositoryModule` — all repository definitions
- `UtilityModule` — utilities, analytics, etc.

Keep the existing `RoomModule` and `NetworkModule` as-is. Register all sub-modules from a root `appModules` list.

## Effort
Medium