a11y(milestones): full keyboard operability

[aburex12345]

Add visible focus-visible styles to milestones controls missing rings, and cover tab order plus Enter/Space activation in dedicated tests.

Description

Milestones controls were keyboard-reachable via native buttons/radios/links, but several interactive elements lacked visible focus styles, so keyboard users could not tell where focus was (WCAG 2.4.7).
This PR adds focus-visible rings/outlines to the gaps (sample banner dismiss ×, creation form Cancel/Submit, due-soon dismiss) without changing layout, and adds dedicated keyboard tests for tab order and Enter/Space activation.
Closes #618

Type of Change

• Bug fix (non-breaking change that resolves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactor (no functional change, internal code improvement)
• Documentation update

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Pre-flight Checklist

All items must be checked before requesting review.

• npm run lint passes with no errors
• npm test passes with no failures
• npm run build completes successfully

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Testing & Coverage

• Module test coverage for impacted files meets or exceeds the 95% minimum threshold
  (run npm test -- --coverage and check the per-file table)
• If this PR adds or modifies UI components, accessibility tests were written using
  the shared utilities in src/test-utils/a11y.tsx

What was tested?

Automated (new suite src/components/__tests__/MilestonesKeyboard.test.tsx):

• Visible focus-visible / focus-within classes on Add Milestone, sample dismiss ×, Start from scratch, form Cancel/Submit, filter labels, due-soon dismiss + links
• Enter/Space: open create dialog, dismiss sample banner, Cancel form, select filter radio, dismiss due-soon (focus moves to scroll region)
• Tab order: filter radios → Add Milestone → due-soon link → dismiss → scroll region
• Arrow keys within MilestoneFilter radiogroup
• jest-axe on populated page and creation form
  Coverage (impacted modules): Statements 95.74%, Lines 95.62%
  Full suite: 74 suites / 1279 tests passed.

Full test output

Test Suites: 74 passed, 74 total Tests: 1279 passed, 1279 total Snapshots: 7 passed, 7 total Time: 18.645 s

MilestonesKeyboard suite: 19 passed.

Accessibility & Security Notes

Accessibility

• Added visible focus-visible outlines/rings on controls that previously had hover-only styling.
• Native controls remain operable with Tab, Enter, and Space; radiogroup supports arrow-key selection.
• Automated checks: jest-axe in MilestonesKeyboard.test.tsx (plus existing milestones axe coverage). Focus-visible assertions mirror ActionPanel / EmptyState patterns.
• Manual: keyboard tab order and Enter/Space activation covered in tests; no screen-reader pass beyond axe.

Security

N/A — no auth, wallet, API, or new user-input handling; styling and a11y tests only.

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