Enable Biome a11y lint rules (staged, after the unlabeled-controls sweep)

[atomantic]

Enable Biome's a11y lint rule group in client/biome.jsonc (confirmed no a11y config present today) so this class of accessibility gap (unlabeled form controls, missing alt text, invalid ARIA props/roles) is enforced automatically instead of relying on periodic manual audits.

This got cheaper when the client moved from ESLint to Biome: the a11y rules ship in Biome itself, so this is purely a config change — no new dependency needed.

Fix shape

Still gated on #4156 (sweep remaining unlabeled form controls) landing first — enabling the full a11y group today would surface hundreds of findings at once and fail CI until that backlog is drained. Sequence:

1. Land the unlabeled-controls sweep (Sweep remaining unlabeled form controls outside FormField #4156).
2. Enable only the rules the repo already substantially honors: a11y/useAltText, a11y/useValidAriaProps, a11y/useAriaPropsForRole, a11y/noLabelWithoutControl — rather than the whole a11y group at once.
3. Expand to the remaining a11y rules in follow-up passes once each is verified clean (or fixed) against the current codebase.

Migrated from PLAN.md by /do:replan --issues. Related to #4156.

[atomantic]

Blocked pending #4156: PR #4274 is open and clean, but this issue explicitly requires that unlabeled-controls sweep to merge before enabling the staged Biome a11y rules. Releasing this claim without changes to avoid knowingly breaking CI; remove the needs-input marker once #4156 lands so the staged config can be claimed.

[atomantic]

Verification after #4156 merged found that noLabelWithoutControl still reports 41 existing violations across 28 files, so enabling it now would fail client lint. I am proceeding with the safe staged slice: enable useAltText, useValidAriaProps, and useAriaPropsForRole, fix its one current finding, and leave noLabelWithoutControl for the remaining label-association sweep.