Add Android instrumentation tests for critical UI flows

## Problem

The `androidTest/` directory is empty — there are zero Android instrumentation tests. All existing tests are unit tests running on JVM (with Robolectric for some). This means there is no end-to-end testing of actual UI behavior on a real Android runtime.

## Location

- `app/src/androidTest/` — empty

## Suggested Fix

Add instrumentation tests for the most critical user flows:
1. **Login/signup flow** — user can create account and sign in
2. **Tree capture flow** — user can capture a tree photo and save it
3. **Sync flow** — trees upload successfully when connectivity is available
4. **Dashboard** — correct tree counts and sync status display

Use Compose testing APIs (`createComposeRule`, `onNodeWithText`, etc.) for UI assertions.

## Effort
High