pdf-server: add annotations, download, form fill, and highlight_text

Add PDF annotation system with 7 annotation types (highlight, underline,
strikethrough, note, rectangle, freetext, stamp), text-based highlighting,
form filling, and annotated PDF download using pdf-lib.

- Server: annotation Zod schemas, extended interact tool with add/update/remove
  annotations, highlight_text, and fill_form actions
- Client: annotation layer rendering with PDF coordinate conversion, persistence
  via localStorage (using toolInfo.id key), pdf-lib-based download with embedded
  annotations and form fills, uses app.downloadFile() SDK with <a> fallback
- Model context includes annotation summary

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

Author: ochafik