fix(10-03): transparent field boxes and fixed-size checkbox in FieldPlacer

- preparePdf: remove opaque fill from all field type rectangles (signature,
  initials, checkbox, date, text) — underlying PDF content now shows through
- preparePdf: checkbox draws X lines only (no border rectangle); date draws
  no placeholder at all; text draws nothing (position marker only)
- sign route: remove white overwrite rectangle on date stamp — date text
  draws directly on existing PDF content
- FieldPlacer: suppress resize corner handles for checkbox fields; hide
  "Checkbox" label (too small at 24x24pt); checkbox is fixed-size only

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

teressa-copeland-homes/src/app/api/sign/[token]/route.ts

@@ -191,12 +191,7 @@ export async function POST(
     for (const field of dateFields) {
       const page = pages[field.page - 1];
       if (!page) continue;
-      // Overwrite the amber placeholder rectangle with white background + date text
-      page.drawRectangle({
-        x: field.x, y: field.y, width: field.width, height: field.height,
-        borderColor: rgb(0.39, 0.45, 0.55), borderWidth: 0.5,
-        color: rgb(1.0, 1.0, 1.0),
-      });
+      // Draw signing date directly — no background rectangle so underlying PDF content shows through
       page.drawText(signingDateStr, {
         x: field.x + 4,
         y: field.y + field.height / 2 - 4,  // vertically center