Reflect market filters in URL and announce result counts

[Fading-Dev]

Overview

The search and category filters on /markets lived only in component state, so a filtered view
could not be shared, bookmarked or restored with the Back button, and the FWC26 campaign links lost
their filter context. Nothing announced the filtered result count to screen-reader users.

Both filters now round-trip through the query string — debounced so typing writes one history entry
per pause rather than one per keystroke — the state hydrates from the URL on load with category
validated against the known list, and each change in result count produces a single polite
live-region announcement through the existing useGlobalLiveRegion hook.

Related Issue

Closes #965

Changes

URL as the source of truth

• [MODIFY] app/markets/page.tsx
  • useSearchParams() now hydrates search and category on mount, so
    /markets?q=argentina&category=Football pre-fills both controls and filters the list.
  • An unknown or missing category falls back to all via an isKnownCategory guard instead of
    filtering to an empty grid.
  • router.replace (not push) writes ?q= / ?category= after a 300 ms debounce, so a typing
    session does not add a history entry per keystroke and Back leaves the filtered view intact.
  • The filter state moved behind a React.Suspense boundary, which useSearchParams() opts the
    route into, so the shell still prerenders.

Announcements

• [MODIFY] app/markets/page.tsx
  • Announces N markets found politely through useGlobalLiveRegion() after filtering,
    deduplicating identical counts so re-renders do not repeat the message.
  • NoMatchEmptyState's clear action resets both the component state and the URL.

Tests

• [MODIFY] app/markets/__tests__/page.test.tsx
  • Covers URL hydration of both controls, the invalid-category fallback, the debounced replace
    write, the announcement payload and the clear-button reset.

Verification Results

Static review only — no clone or sandbox is available in this environment, so `pnpm test
app/markets` and `pnpm test hooks/__tests__` were NOT executed and no pass is claimed.
- The debounce timer is cleared on unmount, and the URL write uses `replace`, so the history
  criterion is enforced by construction.
- Announcements are deduplicated by comparing against the last announced count.

Acceptance Criteria	Status
Typing a search updates ?q= in the URL without adding a history entry per keystroke	✅ 300 ms debounce + router.replace
Loading /markets?q=argentina&category=Football pre-fills both controls and filters the list	✅ state initialised from useSearchParams()
An invalid category param falls back to all	✅ isKnownCategory guard against the CATEGORIES list
Each change in result count triggers one polite live-region announcement	✅ useGlobalLiveRegion() with duplicate-count suppression
app/markets/__tests__/page.test.tsx covers URL hydration and the announcement	✅ tests extended; ⚠️ not executed locally

Closes #965

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