fix(frontend): add focus management to wallet modal

[zainabwahab-eth]

Closes #1257

Summary

frontend/src/components/wallet/WalletModal.tsx implemented its own Escape handler and body-scroll lock but had no focus trap and no focus restoration. Tab walked straight out of the open dialog into the page behind it, and closing the modal dropped focus on document.body.

This PR migrates the modal to the repository's existing useModalDialog hook, the same one already used by TopUpModal, CancelConfirmModal, StreamDetailsModal, StreamCreationWizard and the navbar's MobileMenu. No new focus-trap mechanism was written.

Accessibility changes

• useModalDialog({ onClose, isCloseDisabled: isConnecting }) replaces the bespoke useCallback + document.addEventListener("keydown", ...) Escape handler and the standalone body-scroll-lock effect.
• dialogRef is attached to the inner .wallet-modal panel, matching how TopUpModal and CancelConfirmModal place it (role="dialog" stays on the backdrop).
• Focus now moves into the dialog on open, Tab wraps from the last focusable back to the first, Shift+Tab wraps from the first back to the last, and closing restores focus to the element that opened the modal.
• isCloseDisabled is wired to isConnecting, preserving the existing rule that the modal cannot be dismissed mid-connection. That already matched the disabled close button and the guarded backdrop click.

Wallet selection, connect/clearError calls, the Freighter detection poll, the install-link fallback, error banner, props, state and styling are untouched.

Files changed

• frontend/src/components/wallet/WalletModal.tsx (+12 / -25)
• frontend/src/components/wallet/WalletModal.test.tsx (new)

Tests run and actual results

New WalletModal.test.tsx covers the acceptance criteria directly, using @testing-library/user-event against a harness with a real trigger button:

1. focus moves into the dialog on open
2. Tab through a full cycle plus one, asserting focus never lands on the trigger or document.body, and wraps last -> first
3. the same for Shift+Tab, wrapping first -> last
4. Escape closes and focus returns to the trigger (after first asserting focus had actually left it)
5. body scroll locks on open and releases on close
6. Escape is ignored while status === "connecting"

All commands run from frontend/:

npx vitest run src/components/wallet/WalletModal.test.tsx src/hooks/useModalDialog.test.tsx src/__tests__/wallet-entry.test.tsx
    -> 3 files passed, 15 tests passed

npx vitest run
    -> 27 files passed, 239 tests passed

npm run lint
    -> 0 errors, 1 pre-existing warning in src/components/dashboard/ActivityHistory.tsx (unrelated)

npm run build
    -> Compiled successfully, TypeScript finished, 13/13 static pages generated

Verification that the tests catch the actual bug: with WalletModal.tsx stashed back to its pre-migration state, tests 1, 2 and 3 fail (3 failed | 3 passed). They pass with the migration applied.

Acceptance criteria

• WalletModal uses useModalDialog
• Bespoke Escape handling removed (the hook provides it, gated by isCloseDisabled)
• Bespoke body-scroll locking removed (the hook provides it)
• Tab cannot move focus outside the modal
• Shift+Tab cannot move focus outside the modal
• Closing the modal restores focus to the triggering element
• Keyboard-navigation test added and passing
• Existing useModalDialog and wallet-entry tests still pass; full frontend suite green (239 tests)
• Lint and next build pass

Pre-existing issues found, not fixed here

Out of scope for this PR, recorded rather than touched:

• npx tsc --noEmit reports 27 errors, all in existing test files (src/lib/api/streams.test.ts, src/hooks/useIncomingStreams.test.tsx, src/app/streams/create/__tests__/create-stream-content.test.tsx). They predate this branch; next build does not type-check those files, so CI is unaffected.
• src/components/dashboard/ActivityHistory.tsx:26 emits a react-hooks/incompatible-library lint warning.

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