fix: reject media smart fields as text sources [ANK-38]

[piazza-bot]

Summary

• Filter TTS/image Smart Fields out of valid prompt source fields, including unsaved prompt-map state, persisted Smart Fields, global Smart Fields, and non-canonical field casing.
• Reject invalid media-field prompt references at local-server Smart Field save time.
• Reject invalid typed media-field prompt references in the custom text prompt helper before generation.
• Preserve deck-specific target override behavior by letting target-field callers opt out of global source filtering.

Verification

• source .venv/bin/activate && ./scripts/build.sh check
• source .venv/bin/activate && python -m pytest (139 passed)
• Targeted: source .venv/bin/activate && python -m pytest tests/test_prompt_helpers.py tests/test_local_server.py (62 passed)
• Automated review-agent loop: initial passes found direct local-server/custom-prompt gaps, global filtering semantics, TTS default-source selection, and casing issues; after fixes, the final 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 the change is validation/list filtering behavior rather than a visual layout change; it is covered by targeted UI-helper unit 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-38