Variant sales price — locked / read-only (prod)

Gate 2 3/3 passed · v0.4 9/9 · 51/51 regression. Narong: the per-variant sales price is irrelevant — the POS always prices as product base + modifier extra, and editing a variant's own price did nothing (same as Odoo). So it's now read-only everywhere: the Variants table cell shows the price with a 🔒 (no longer an editable input), and the variant detail page's Sales Price field is disabled with a note pointing to the product base price + the modifier's extra. The product's own base price (single-variant product page) and the variant Cost stay editable. The variant server actions also ignore priceUsd as defence-in-depth. nix-cafe 379f4ce, no migration.