perf(clipboard): make history writes async

[JustYannicc]

Summary

• Replace synchronous full-history clipboard JSON writes with a debounced async queue.
• Persist via same-format pretty JSON using temp-file + rename atomic replacement.
• Keep in-memory clipboard history updates immediate, while clear/delete/stop/quit/update restart flush pending writes.
• Add focused regression coverage for coalesced writes and flush wiring.

Before / After Evidence

• Baseline before implementation: node --test scripts/test-clipboard-history-persistence.mjs reported [clipboard-baseline] rapidAdditions=100 syncWrites=100 serializeMs=3.23 writeMs=25.38 blockingMs=28.61 finalBytes=17372 sourceSaveCalls=10 historyWriteFileSyncSites=1.
• After implementation: focused regression reports [clipboard-after] rapidAdditions=100 coalescedWrites=1 serializeMs=0.05 writeMs=0.33 blockingMs=0.38 finalBytes=17372.

Validation

• node --test scripts/test-clipboard-history-persistence.mjs
• node --test 'scripts/test-*.mjs'\n- ./node_modules/.bin/tsc -p tsconfig.main.json --noEmit\n- node /opt/homebrew/lib/node_modules/npm/bin/npm-cli.js run build:main\n- Codex LSP diagnostics on src/main: 0 errors\n- node scripts/check-i18n.mjs exited 0; existing Italian locale gaps remain (settings.ai.whisper.vocabulary, settings.extensions.appSearchScope)\n\n## Risks\n- If an async save fails, the queue logs the error and leaves history dirty for a later retry, matching the old non-throwing behavior while avoiding main-process sync I/O.\n- Normal app quit may briefly defer while a pending clipboard write drains; updater restart flushes before quitAndInstall.

[JustYannicc]

Superseded by consolidated upstream PR #534 (perf(mac): reduce native polling and async IO stalls), which carries this change together with the related macOS/native/audio/camera async IO performance fixes. Closing this source PR so review can continue on #534.