fix: save custom media through editor note [ANK-350]

[piazza-bot]

Summary

• Save custom text/TTS/image results into the live editor note when available, instead of writing to a fresh card.note() instance.
• Reload the editor with focus preserved after custom field saves.
• Add focused tests for existing-note persistence and add-flow/new-note behavior.

Verification

• source .venv/bin/activate && ./scripts/build.sh check
• source .venv/bin/activate && python -m pytest (135 passed)
• Targeted: source .venv/bin/activate && python -m pytest tests/test_field_menu.py (2 passed)
• Automated review-agent loop: first pass returned there are no issues.
• Anki sandbox startup: ./scripts/build.sh sandbox-local launched Anki 25.07.5 with this branch loaded; Smart Notes loaded, migrations ran, and the local server started on 127.0.0.1:8766.
• Local server health check: curl -s http://127.0.0.1:8766/ping returned { "ok": true }.

No screenshot included because this is editor propagation/persistence behavior rather than a visual layout change; it is covered by focused callback tests and the Anki startup smoke.

Agent session metadata

• Working directory: /Users/michaelpiazza/development/smart_notes_all
• Session ID: 019e9d01-400c-7552-8264-a71ce3a12be5

[linear]

ANK-350