perf(frecency): memoize sorting defaults

[JustYannicc]

Summary

• Hoists the default useFrecencySorting key resolver so stable rerenders do not invalidate useMemo.
• Captures one frecency scoring timestamp per sort recompute instead of calling Date.now() for each score.
• Adds a node test harness for stable rerenders, timestamp count, ordering, visit tracking, and reset behavior.

Evidence

• Baseline before the production change: node --test scripts/test-use-frecency-sorting.mjs failed with stable rerender comparisons increasing 4 -> 8, and one sort reading Date.now() 8 times instead of 1.
• After the change: node --test scripts/test-use-frecency-sorting.mjs passes all 3 tests.

Validation

• npm test passes: 27 passed, 1 skipped.
• npm run check:i18n exits 0; it still reports pre-existing Italian locale gaps for settings.ai.whisper.vocabulary and settings.extensions.appSearchScope.
• npm run build:main passes.
• npm run build:renderer passes.
• LSP diagnostics for the edited hook report no diagnostics.

Risks

• The default key now stringifies id values explicitly; object property access already coerced those keys for persisted records, so existing storage keys should continue to match.

[JustYannicc]

Superseded by the consolidated upstream PR #535: #535