Add unit tests for PositionsViewModel.sortedPositions

[whtis]

Why this is useful

PositionsViewModel.sortedPositions has four sort modes (MarketValue, UnrealizedPnl, DailyPnl, Symbol) with no test coverage. A regression in sort order would silently ship.

Where to start

Create android/app/src/test/java/com/tis/ibkr/viewmodel/PositionsViewModelTest.kt. Construct a list of fake Position instances with varied PnL and market values, set the sort mode, assert order.

Definition of done

• One test per sort mode (4 tests).
• All pass under ./gradlew test.
• No new dependencies (use the existing JUnit on the test classpath).