feat: add useTransactionFilters hook, improve sidebar a11y, and refactor multisig state

[boys-cyberhub]

Closes #939
Closes #940
Closes #941
Closes #942

What changed

#941 — useTransactionFilters hook with optimistic updates (frontend/src/hooks/useTransactionFilters.ts)

• Implemented the missing hook (the file previously contained test code; this replaces it with the actual implementation)
• Uses useReducer + paymentHistoryFiltersReducer so draft filter state updates synchronously on every interaction — no waiting for URL sync
• Search: debounced 350 ms before pushing to URL; searchSyncPending stays true in the meantime so callers can show a syncing indicator without blocking the input
• Non-search filters: pushed inside a React useTransition; isFilterPending exposes the transition state
• Debounce is cancelled on onClearFilter("search") and onClearAll so no stale push fires after clearing
• hasActiveFilters reflects the optimistic draft state immediately

#940 — Screen reader improvements (frontend/src/components/TransactionFilterSidebar.tsx)

• Desktop sticky panel: added role="complementary" and aria-label="Transaction filters" so it is a named landmark for AT users
• SyncSpinner wrapper inside active asset buttons: added aria-hidden="true" so the spinner text ("Syncing…") is excluded from the button's computed accessible name
• Clear All button: aria-label now includes the active filter count, e.g. "Clear all 2 active filters"
• Added a role="status" / aria-live="polite" / aria-atomic="true" region in the header that announces count changes to screen readers (e.g. "2 filters active" → "1 filter active")

#941 — Optimistic active-filter count badge (frontend/src/components/TransactionFilterSidebar.tsx)

• Count badge (aria-hidden) in the header shows the number of active filter dimensions and updates before URL sync (driven by the optimistic filters prop, not the committed URL state)

#939 — Extended unit tests (frontend/src/components/TransactionFilterSidebar.test.tsx)

• Added section 7 · Screen reader & optimistic updates (13 new test cases) covering:
  • Desktop panel landmark role and label
  • aria-hidden on SyncSpinner wrapper inside asset buttons
  • Count badge absent / singular / multi-filter
  • Live region empty when no filters, singular/plural announcement text
  • Clear All aria-label with count vs. generic label
  • Mobile dialog retaining its role="dialog" and aria-modal attributes

#942 — MultisigProvider refactored to useReducer (frontend/src/lib/multisig-context.tsx)

• Replaced 6 separate useState calls with a single useReducer + multisigReducer
• Action types: SET_TRANSACTION, SET_STEP, SET_LOADING, SET_ERROR, CLEAR_ERROR, RESET, SET_VISIBLE, OPTIMISTIC_SIGN, CONFIRM_SIGN, REVERT_SIGN, SUBMIT_SUCCESS
• OPTIMISTIC_SIGN sets hasSigned: true immediately (before async); CONFIRM_SIGN atomically attaches the real signature and auto-advances to the "submit" step if the weight threshold is met
• signTransaction and submitTransaction use a stateRef (always-current snapshot) to avoid stale-closure issues without needing the state in their dependency arrays
• All public APIs unchanged: useMultisig(), useMultisigState(), useMultisigActions(); all existing tests in multisig-optimistic.test.tsx continue to pass

How to test

cd frontend
npm run test -- TransactionFilterSidebar
npm run test -- useTransactionFilters
npm run test -- multisig-optimistic

• Verify the desktop sidebar has a complementary landmark in browser accessibility tree (DevTools → Accessibility pane)
• Verify active filter count badge updates instantly (no URL round-trip delay) when toggling filters
• Verify screen reader announces count changes as filters are applied/cleared

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