perf(oracle): throttle visibilitychange refetches in useOracle hooks (#519)

[oluwarantimini]

Summary

src/hooks/useOracle.ts refetched every time the tab became visible. Switching tabs quickly (10 times in 5 seconds, for example) sent one oracle API request per switch, even though the hook already polls every ORACLE_REFRESH_INTERVAL_MS (60s).

Both hooks had this problem: useOracleReading and useAllOracleReadings, which backs the oracle page.

Fix

This follows the approach suggested in the issue:

• Added VISIBILITY_REFETCH_MIN_MS = ORACLE_REFRESH_INTERVAL_MS / 2 (30s by default). It is exported so tests can use it.
• Each hook stores the time of its last fetch in a ref (lastFetchAtRef). The ref is updated at the start of every load(), whether the fetch came from the first load, the polling interval, a visibility change or a manual refetch.
• The visibilitychange handler now refetches only if the tab is visible and the last fetch was at least VISIBILITY_REFETCH_MIN_MS ago.

Unchanged:

• Interval polling is still the main way data refreshes.
• Manual refetch() is not throttled, so a user who clicks refresh always gets fresh data.
• The listener cleanup from [Bug] useOracleReading visibilitychange listener persists across pages #471 and the stale-response guard from [Bug] useAllOracleReadings has no stale-response protection #450 are untouched.

Tests

The new tests are in src/hooks/__tests__/useOracle.test.ts. They stub Date.now and dispatch real visibilitychange events:

• useOracleReading: 10 rapid visibility changes trigger no extra fetch. After moving the clock past VISIBILITY_REFETCH_MIN_MS, one more change triggers exactly one refetch.
• useAllOracleReadings: the same check.

Results:

npx vitest run src/hooks/__tests__/useOracle.test.ts
 Test Files  1 passed (1)
      Tests  9 passed (9)

• With the old useOracle.ts, both new tests fail (2 failed, 7 passed), so they do catch the bug.
• tsc --noEmit reports no errors in the changed files. The errors it does report are in unrelated files and were already there.
  Closes [perf] Oracle readings refetch on every visibility change without debounce #519
  Closes [ux] Category filter state lost on page navigation #518
  Closes [bug] Race condition in useOracleReading when key changes rapidly #516
  Closes [bug]科学计数法输入可以绕过覆盖范围验证 #517

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