pdf-server: add get_pages tool for batch text/screenshot extraction

New tool `get_pages` lets the model get text and/or screenshots from
arbitrary page ranges without navigating the visible viewer.

- Server: `get_pages` tool with interval-based page ranges (optional
  start/end, open ranges supported), `getText`/`getScreenshots` flags,
  request-response bridge via `submit_page_data` app-only tool
- Client: offscreen rendering (hidden canvas, no visual interference),
  text from cache or on-demand extraction, screenshots scaled to 768px
  max dimension, results submitted back to server
- Max 20 pages per request, 60s timeout

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

Author: ochafik