feat: unit-aware ingredient quantities in recipes

[johncarmack1984]

Product feedback (7/23): recipes force grams and produce nonsense — "1 g Rudi's Whole Wheat Bun".

The recipe editor spoke only grams and prefilled a new line with the ingredient's serving grams shown as grams. Nutrition math was always fine (everything is canonical grams); the gap was the entry/display unit.

Model — the ingredient's serving IS its count unit

The amounts table already carries unit / amount (count) / grams (canonical) / preferred. Grams stays the single source of truth for all nutrition math, so units are a pure display/entry layer.

• An ingredient can name its serving unit ("bun", "slice", "patty", "each"; default "serving"), stored on the existing serving amount — no schema change. The ingredient form gains the field.
• Recipe lines get a unit dropdown: the ingredient's count unit (its serving) + universal mass units (g, oz). A new line defaults to "1 " when a serving is declared, else 100 g. The client computes grams = count × factor; nutrition math is untouched.
• Lines persist count + unit + preferred, so the detail reads "2 buns" and the form reloads in the same units. The content pull carries the count, so the local-first desktop shows the author's units after a sync (the web reads the server directly).

Migration decision (existing rows — documented, not reinterpreted)

Existing lines were stored with amount == grams, a "g" label, preferred = 'grams'. They render exactly as before — as grams. We do NOT retro-guess that a stored "1 g" meant "1 unit" (no reliable signal; silently rescaling would corrupt real gram amounts). Only new/edited lines use the count model — re-editing a line lets you switch it to a count unit.

Scope

Shipped: serving-unit data model + ingredient-form field + unit-aware recipe form + storage + recipe-detail display + content-pull fidelity. Deferred (documented, non-regressions — they stay grams, which is correct): the inline detail-editor's in-place amount scrubbing and the log add-flow keep gram entry; both can adopt the same picker next.

Verification

• Full just check green — lint, typecheck, web tests, web + desktop builds, all Rust tests, schema-parity + bindings drift.
• Live round-trip against a throwaway-DB server: create an ingredient with a "bun" serving unit (52 g) + a recipe of "2 bun", then confirm the detail (amount 2 / unit bun / grams 104), the edit reload (preferred: units), and the content pull (amount: 2) all carry the count with correct grams.