Tech: Focus controller for deterministic row navigation (Editor Mode)

[GitAddRemote]

Epic: #58

Problem

React re-renders, sorting, grouping, and pagination can cause focus loss or jumps. Editor Mode requires deterministic, testable focus movement to mimic spreadsheet workflows.

Acceptance Criteria

• Centralized focus controller for Editor Mode can:
  • Move focus Location → Quantity → Save → next row Location.
  • Target a specific row+field by stable row key.
  • Handle pagination boundary (last row save → first row next page) when possible without jank.
• No focus jank when:
  • Rows re-render due to state/optimistic updates.
  • Sorting/grouping changes order (document expected behavior when order changes mid-edit).
• Does not break screen reader or tab order; remains accessible.

Technical Notes

• Provide an imperative API (e.g., registry mapping row keys to field refs) to request focus; keep registration/unregistration stable across renders.
• Compute next target from the rendered order used by grouping/sorting/pagination; avoid stale indices.
• Defer focus until after state updates where necessary (e.g., requestAnimationFrame/microtask) to prevent focusing unmounted elements.
• Integrate with the save flow (Inventory Editor Mode: Per-row save without reload + saved indicator #63) to avoid focus loss during in-flight saves/spinners.
• Expose hooks/utilities for tests to assert current focus target.

Definition of Done

• Focus controller implemented and used by Editor Mode flows; ad-hoc ref juggling removed where possible.
• Focus transitions are deterministic in personal + org views, including pagination edge cases or documented fallback.
• A11y preserved (tab order and SR navigation still work).
• Tests updated/passing to assert focus behavior; lint/typecheck/CI clean.