web: replace NamedEntityPicker's combobox with MUI Autocomplete #826

Part of #674. The single largest win on this epic.

NamedEntityPicker is 1,147 lines with 1,852 lines of test across four files, plus the
CustomerPicker / FlockPicker adapters. It is a combobox: keyboard, focus and announcement
behaviour that is a solved problem with a spec.

Replace the control layer with MUI Autocomplete, which supports async search and paging.

What does NOT come for free

The async discovery engine is this app's own and stays: discovery generations, the offset cursor
driven by raw server row counts, eligibility policy (FR-009), retention, the US2/US3 selection
transitions, unavailable states. Scope this as replacing the combobox, not the engine.

Constraints

• Every string still ships en/es/tl; tl runs ~30% longer (i18n: catalogParity compares key SETS, so a translation that drifts in MEANING passes every gate #688). No baked-in English.
• Colours resolve through CSS custom properties — FarmThemeProvider already carries this.
• Preserve the PWA/shell: verify the inert-swallowed announcements with a real screen reader #501 accessibility behaviour; anything reasoning about inert or the AX tree needs
  CDP, not Playwright's own APIs.