a11y(wallet): manage focus on transitions

[Carlys17]

Summary

Adds proper focus management on wallet route/dialog transitions so keyboard and screen-reader users aren't disoriented.

Changes

• src/hooks/useWalletFocus.ts — new hook that moves focus to connected element on connect and restores focus to connect button on disconnect
• src/components/WalletConnectButton.tsx — uses useWalletFocus to manage focus on connect/disconnect transitions
• src/components/HeaderActions.tsx — mobile toggle focuses first interactive element on open, restores focus to toggle button on close
• src/hooks/tests/useWalletFocus.test.tsx — tests for the hook (connect, disconnect, isConnecting, rapid cycles)
• src/components/tests/a11y.test.tsx — focus-behaviour tests for WalletConnectButton + HeaderActions

Acceptance criteria coverage

• Move focus to sensible target on wallet open/route-change
• Restore focus on close/disconnect
• Trap focus in wallet dialogs (existing useDialogFocusTrap already handles ConfirmDialog + MilestoneCreationForm)
• No visual change — focus behavior only via refs + useEffect
• Focus-behaviour tests added
• npm run lint clean
• npm test — 1270 tests pass
• npm run build succeeds

Test output

Test Suites: 74 passed, 74 total
Tests: 1270 passed, 1270 total
Snapshots: 7 passed, 7 total
Time: 81.763 s

Closes #692

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