feat(recipe): scale ingredient quantities (½×, 1×, 2×, 3×)

[dmnyc]

Summary

Add a scale selector to the recipe's Ingredients section so cooks can double, triple, or halve a recipe on the fly. Chosen scale persists per-recipe in localStorage (recipe_ingredients_scale:<id>) and only appears in the UI when ≠ 1×; when active a caption notes that directions and cook times are unchanged (they don't scale linearly with batch size).

Parsing

The new $lib/ingredientScaling.ts utility handles the common ingredient-line shapes:

• whole / decimal / fraction / mixed number / unicode fraction — 2, 0.5, 1/2, 1 1/2, ½
• optional range — 2-3 cloves, 1/2 to 1 cup

Non-parseable lines pass through unchanged so entries like "salt to taste" or "a pinch of cinnamon" are never mangled. Output reformats to nice unicode fractions (½, ⅓, ⅔, ¼, ¾, ⅛, ⅜, ⅝, ⅞) when the scaled value lands within tolerance, else a short decimal.

Examples

Original	2×	3×
2 cups flour	4 cups flour	6 cups flour
1/2 tsp salt	1 tsp salt	1 ½ tsp salt
1 1/2 cups milk	3 cups milk	4 ½ cups milk
1/3 cup sugar	⅔ cup sugar	1 cup sugar
2-3 cloves garlic	4–6 cloves garlic	6–9 cloves garlic
salt to taste	unchanged	unchanged

Notes

• Stacked on Recipe display: flat directions, tickable ingredient & direction checkboxes #308 → fix(post-composer): restore mention autocomplete suggestions #306 → fix(error-boundary): stop showing fatal UI for benign window errors #305 → Recipe editor UX: auto-save, add-by-URL, drag-reorder, markdown links #304. Once those merge, this PR's diff collapses to the new ingredientScaling.ts plus a small Ingredients.svelte header update.
• Parser lives in a pure utility so future surfaces (print modal, grocery list export) can reuse it.

Test plan

• Set scale to 2× / 3× / ½× → quantities update in place; unicode fractions render correctly.
• Reload — chosen scale persists per recipe.
• Switch recipes — scale is recipe-specific, not shared.
• Scale a recipe with a ranged ingredient (2-3 cloves) — both ends scale.
• Scale a recipe containing non-numeric entries (salt to taste, pinch of cinnamon) — those pass through unchanged.
• Printing still shows the original (unscaled) ingredient list (PrintRecipeModal reads from event.content directly).