feat(3d): meal-box merge, stored mutations, undo, mobile undo UI + i18n ×12

- Merge leftovers into existing meal_box when same recipeId + cookedAt date + frozen + available.
- Store mealBoxMutations JSONB on cooking_sessions for deterministic undo.
- Undo decrements portions/remaining, discards box at zero, appends correction ledger rows.
- Mobile: undo button in cooking/[id].tsx after-flow and meal-boxes.tsx within 24h window.
- i18n undo strings across all 12 locales; parity test green.
- 6 new integration tests: merge, no cross-date merge, frozen split, undo restore, undo discard, ledger invariant.
- Update FAS3 audit doc with 3d semantics.

Closes Fas 3d

infrastructure/migrations/0015_meal_box_leftover_source.sql

@@ -0,0 +1,4 @@
+-- Fas 3d: rester från cooking sessions spåras i befintliga meal_boxes.
+ALTER TABLE meal_boxes ADD COLUMN IF NOT EXISTS leftover_source TEXT;
+-- Bakåtfyll: befintliga matlådor skapade före 3d får källan 'cook_session'.
+UPDATE meal_boxes SET leftover_source = 'cook_session' WHERE leftover_source IS NULL;