fix: warn on dusty position metrics

[miohtama]

Why

Dust residuals can make closed position metrics look economically meaningful even when the remaining balance is too small to claim or trade. Position pages need to warn users when metrics may be distorted by this state.

Lessons learnt

Closed positions can end with a non-zero residual quantity and near-zero recorded value without an explicit close trade flag, so the frontend needs a generic residual-value dust signal in addition to explicit dust notes.

Summary

• Add a dust warning predicate to enriched trading positions.
• Show a warning alert below the position page heading when the position looks dusty.
• Rename the position trade table heading from Trades to Rebalances.