Add a visual multi-page editor for PdfLetter fields

[Copilot]

This introduces a visual editor for PdfLetter field placement so fields can be added, removed, and adjusted directly against the source PDF. It also exposes multi-page positioning and field inventory data needed to manage repeated merge fields from a single UI.

• Editor payload and field persistence

  • Added editor-facing serialization on PdfLetterTrait for:
    • PDF page dimensions
    • embedded PDF preview source
    • existing placed fields
    • available merge fields with placement counts
  • Added field sync support to create, update, and delete editor-managed fields in one request.

• Per-page field positioning

  • Extended FieldTrait with explicit page metadata, editor serialization, and editor input normalization.
  • Preserved compatibility with existing absolute-Y positioning by inferring page from legacy coordinates when page metadata is absent.
  • Updated PDF rendering helpers and writers to honor explicit page selection for text and barcode fields.

• Visual editor surface

  • Added PdfLetterFieldsEditorControllerTrait with endpoints for:
    • rendering the editor
    • loading editor data
    • saving field layouts
  • Added library-provided editor template, CSS, and JS assets for:
    • field palette/sidebar
    • per-page canvas overlays
    • drag-based placement
    • inline property editing
    • field removal and layout save

• Field editing model

  • Extended admin field form support with page input alongside existing size, color, align, and transform controls.
  • Normalized editor color input to the storage format already used by PDF rendering.

• Example of the new editor contract

  $payload = $letter->getEditorPayload();
  
  // pages, preview source, placed fields, and available merge fields
  $fields = $letter->syncEditorFields($request['fields']);

• Behavioral coverage

  • Added tests for page inference, editor coordinate normalization, and explicit-page PDF Y-position handling.