feat: server-side PPTX conversion + screenshot dashboard system
PPTX Conversion:
- Remove LibreOffice from client dependencies (server uses Gotenberg)
- Update docs to reflect clients only display pre-rendered PDFs
- Clarify server-side conversion workflow in README and copilot-instructions
Screenshot System:
- Add background screenshot capture in display_manager.py
- Implement Wayland/X11 session detection with tool fallback
- Add client-side image processing (downscale + JPEG compression)
- Create timestamped files + latest.jpg symlink for easy access
- Implement file rotation (max 20 screenshots by default)
- Enhance simclient.py to transmit via MQTT dashboard topic
- Add structured JSON payload with screenshot, timestamp, system info
- New env vars: SCREENSHOT_CAPTURE_INTERVAL, SCREENSHOT_MAX_WIDTH,
SCREENSHOT_JPEG_QUALITY, SCREENSHOT_MAX_FILES, SCREENSHOT_ALWAYS
Architecture: Two-process design with shared screenshots/ volume
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