Add gains by tax treatment to investment report with grouped subtype dropdown (#701)

* 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>

app/helpers/reports_helper.rb

@@ -1,4 +1,18 @@
 module ReportsHelper
+  # Returns CSS classes for tax treatment badge styling
+  def tax_treatment_badge_classes(treatment)
+    case treatment.to_sym
+    when :tax_exempt
+      "bg-green-500/10 text-green-600 theme-dark:text-green-400"
+    when :tax_deferred
+      "bg-blue-500/10 text-blue-600 theme-dark:text-blue-400"
+    when :tax_advantaged
+      "bg-purple-500/10 text-purple-600 theme-dark:text-purple-400"
+    else
+      "bg-gray-500/10 text-secondary"
+    end
+  end
+
   # Generate SVG polyline points for a sparkline chart
   # Returns empty string if fewer than 2 data points (can't draw a line with 1 point)
   def sparkline_points(values, width: 60, height: 16)