Canteen Management

Adds a "Sell at the counter" section to the recipe create/edit dialog (Staff price, Visitor price, Menu category, GST, Vegetarian). Recipe cost itself is unchanged -- still computed live from ingredients x current store rate, never hand-typed, per the modules documented costing invariant.

The new fields write to a MenuItem, a separate entity, not the recipe:

• New recipe, or an existing one with no linked menu item yet: filling in both prices creates a MenuItem on save.
• Existing recipe with exactly one linked menu item: prices pre-fill from it and editing+saving updates that item.
• Existing recipe linked to more than one menu item: section is hidden -- the price is ambiguous, use Menu and pricing instead.
• Hidden entirely for a user without canteen.menu_items.update (e.g. Kitchen Supervisor), and for a therapeutic recipe (never sold at the counter).

Verified end to end in a live browser session: created two test recipes with prices, confirmed the matching MenuItem appeared correctly in Menu and pricing with the right staff/visitor price, category and GST; confirmed the edit path pre-fills from an existing single menu item (tested against the real seeded Idli recipe, cancelled without saving). Test recipes/menu items archived afterward.