* Add tax treatment metrics to reports, forms, and models
- Implement `build_gains_by_tax_treatment` for grouping gains by tax treatment
- Update investment performance view with tax treatment breakdown
- Add tax treatment field to crypto and investments forms
- Introduce `realized_gain_loss` calculation in the Trade model
- Group investment subtypes by region for improved dropdown organization
* Optimize investment performance report by reducing N+1 queries
- Eager-load associations in `build_gains_by_tax_treatment` to minimize database queries
- Preload holdings for realized gain/loss calculations in trades
- Refactor views to standardize "no data" placeholder using translations
- Adjust styling in tax treatment breakdown for improved layout
* Enhance investment performance translations and optimize holdings lookup logic
- Update `holdings_count` and `sells_count` translations to handle pluralization
- Refactor views to use pluralized translation keys with count interpolation
- Optimize preloaded holdings lookup in `Trade` to ensure deterministic selection using `select` and `max_by`
* Refine preloaded holdings logic in `Trade` model
- Treat empty preloaded holdings as authoritative to prevent unnecessary DB queries
- Add explicit fallback behavior for database query when holdings are not preloaded
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Co-authored-by: luckyPipewrench <luckypipewrench@proton.me>
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