[R1-01] Verify and finish per-paper Notes workflow

[dkritarth]

Outcome

Deliver reliable per-paper research notes with autosave and restart persistence.

Dependencies

Blocked by #28. Existing PR #21 is implementation candidate, not completion proof. Do not merge it until this issue is ready and all criteria have current evidence.

Scope

Review PR #21 against current master and this issue. Verify migration, repository, IPC validation, preload typing, editor states, autosave, switching papers, clearing note, and restart persistence. Fix defects on same focused branch or follow-up PR.

Acceptance criteria

• Note is scoped to active paper and never appears on another paper.
• Typing autosaves without losing last debounced edit on tab/paper/app change.
• Empty and loading states are clear.
• Clear action is deliberate and persisted.
• Note survives full app restart.
• Deleting paper cascades note safely.
• Migration works from current production schema and fresh DB.
• Live console remains clean.

Live verification

Use two real papers. Write distinct notes, switch rapidly, restart app, verify isolation, edit during debounce then close/switch, clear one note, restart again.

Required PR evidence

PR must use Closes #29, stay focused, and include:

• regression/failing test or reproducible baseline;
• focused tests plus npm test, npm run typecheck, npm run build, git diff --check;
• exact Electron clicks/typing and observed result;
• screenshots for UI changes when tooling permits;
• console output, restart/persistence result, and unresolved limitations;
• real-paper evidence for reader behavior.

Existing PR code and mocked tests are not sufficient. Reviewer must independently exercise live flow before merge.