Canteen Management

Removes the live ingredient-computed cost display from the recipe create/edit dialog and the read-only recipe detail sheet -- no more "Batch cost" column, no more live "Cost per portion" footer total. The ingredient list stays (still needed for stock/GRN tracking), and the server still computes and falls back to the ingredient total when no manual cost is set (RecipeDto.computedCostPerPortion, unchanged), but nothing in the UI shows that breakdown or emphasises it anymore.

The "Override cost per portion (optional)" field from #684 is renamed to just "Cost per portion" -- it is now the one, direct, primary way to set what a recipe costs, not a special case layered over a visible computed figure. Updated the surrounding copy (page description, stat hint, sold-as footnote) to stop implying the cost is calculated from ingredients live.

Removed the now-fully-unused frontend preview helpers (bomLineCost, bomTotalCost, BomRowContext) and their tests from recipe-bom.ts -- nothing called them once the computed display was gone. Backend untouched; this is a frontend-only change. Verified live: Edit Dhosha shows Ingredient/Qty/Rate with no cost columns, and a single "Cost per portion" field with no competing auto-calculated number anywhere on screen.