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/0018_cooking_sessions_meal_box_mutations.sql

@@ -0,0 +1,3 @@
+-- Fas 3d: spara vilka meal_boxes som påverkades av en session och med
+-- hur många portioner, för deterministisk undo vid mergeade lådor.
+ALTER TABLE cooking_sessions ADD COLUMN IF NOT EXISTS meal_box_mutations JSONB DEFAULT '[]'::JSONB;