perf(notes): coalesce autosave persistence

[JustYannicc]

Summary

• Replace synchronous Notes JSON writes on each mutation with a debounced async atomic write queue.
• Keep notesCache as the authoritative in-memory state for immediate reads while persistence drains in the background.
• Add flushNotesToDisk() / hasPendingNotesSave() and defer Electron quit only when a Notes save is pending or in-flight.
• Add a focused Notes persistence regression harness that counts physical Notes JSON writes and verifies shutdown flush behavior.

Before / After Evidence

• Baseline before production edits: 20 repeated autosaves produced 20 synchronous writes, loopMs=23.77, syncWriteMs=23.11.
• After: 20 repeated autosaves produce 1 async atomic write, 0 sync writes, latest observed full-suite metric loopMs=2.62, syncWriteMs=0.00.

Validation

• node --test scripts/test-notes-async-saves.mjs
• node --test scripts/test-*.mjs (26 pass, 1 live Electron test skipped because Electron is not launchable here)
• ./node_modules/.bin/tsc -p tsconfig.main.json
• Codex LSP diagnostics for src/main: 0 errors
• node scripts/check-i18n.mjs (exits 0; reports existing Italian locale gaps for settings.ai.whisper.vocabulary and settings.extensions.appSearchScope)
• git diff --check

Remote Checks

• claude-review failed before running review logic because the action could not fetch an OIDC token: Unable to get ACTIONS_ID_TOKEN_REQUEST_URL env variable; log suggests adding id-token: write to workflow permissions.

Risks / Notes

• Persistence now returns to callers before disk rename completes; pending/in-flight writes are flushed on app quit.
• If an async disk write fails, behavior matches the prior non-throwing save path by logging the error and preserving in-memory state.

[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Superseded by consolidated upstream PR #534 (perf(mac): reduce native polling and async IO stalls), which carries this change together with the related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.