fix(13): switch to GPT-4o vision — render PDF pages as images for accurate field placement

- extractPdfText now renders each page to JPEG via @napi-rs/canvas + pdfjs-dist (108dpi)
- field-placement.ts sends rendered page images to GPT-4o with vision (detail: high)
- AI can now visually identify underlines, signature blocks, date fields, initials boxes
- System prompt focuses on visual cues (blank lines, boxes) not text pattern matching
- Handles multi-field lines: separate fields for signature blank and date blank on same line

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

teressa-copeland-homes/package.json

@@ -18,6 +18,7 @@
     "@cantoo/pdf-lib": "^2.6.3",
     "@dnd-kit/core": "^6.3.1",
     "@dnd-kit/utilities": "^3.2.2",
+    "@napi-rs/canvas": "^0.1.97",
     "@react-email/components": "^1.0.10",
     "@react-email/render": "^2.0.4",
     "@vercel/blob": "^2.3.1",