web+api: per-grade low-stock floors so the Dashboard's morning brief and stock ledger can warn #911

Component plan: build this from the components named in docs/designs/822-mui-revamp.md. The rule is "reused, never rebuilt" — composing MUI primitives into a screen wrapper is expected; reimplementing behaviour a shared component already has is not. Say in the PR which row of that table covers this screen.

Part of #674, Phase 3 follow-up. Filed by the owner during #906 (2026-09-19).

Why

The approved Operations Desk direction for the Dashboard (#906, Concept B) shows a low-stock fact in the morning brief and a Cracked row "visibly tied to its floor warning" in the Available Stock ledger. No stock floor exists anywhere today: StockResponse carries Available and Restricted only, the farm settings carry no threshold, and Dashboard.tsx records that a stock-floor attention source was proposed on #864 and not taken. #906 therefore omits both elements and records the omission; this issue supplies the data the design assumed.

Scope

• Setting: a per-grade low-stock floor, in eggs, on the farm's grade catalog (Grades screen, Owner-only like the rest of farm configuration, Farm configuration and identity become Owner-only, matching spec §5.1 #729). Optional per grade; unset means no warning. Stored on the grade, not the account, so a renamed or deactivated grade keeps or drops its floor with it.
• API: StockResponse gains the grade's floor and a BelowFloor flag computed from Available (not Available + Restricted; restricted eggs cannot be sold). No new endpoint.
• Dashboard: the morning brief adds the low-stock fact ("N grades below floor", or the one grade by name when it is one) beside the unresolved-house fact; the stock ledger marks each below-floor row without relying on colour alone (a labelled mark plus text for assistive technology), which is what web: redesign Dashboard around the Operations Desk direction #906's design drew for Cracked.
• Stock page: the same mark on the grade rows.
• Audit: floor changes write through the existing grade-update audit action; no new vocabulary unless review finds one is needed.
• Three locales for every new string; product glossary and Help updated for "low-stock floor" per the docs-in-sync rule.

Behaviour that must not change

The six-read Dashboard pipeline and independent panel degradation; the attention line's one-source rule for houses (this adds a second, separately sourced fact to the brief, not to that line, unless #906's shipped composition merged them, in which case follow #906); stock quantity and restriction semantics; role gates.

Acceptance criteria

• Owner can set and clear a floor per grade; Manager and below cannot.
• GET /stock reports the floor and the below-floor flag; integration test covers unset, above and below.
• Dashboard morning brief and stock ledger, and the Stock page, render the warning from real data; unit tests pin the text and the non-colour mark.
• Before/after captures at 1280x800 and 390x844, light and dark, 1:1, from a rebuilt stack.
• The PR body contains Closes #NNN and the closing link is verified through GitHub's API.

Out of scope

Reopening #906 or #864; per-lot or per-house floors; notifications outside the Dashboard.