Adds pantry_match_only flag to RecipeRequest. When enabled, any recipe
with one or more missing ingredients (after swaps) is excluded from
results. Swapped ingredients count as covered.
Frontend: toggle checkbox in recipe settings panel, disabled when
shopping mode is active (the two modes are mutually exclusive). Hides
the max-missing input when pantry-match-only is on (irrelevant there).
Closes#63