Add vector PDF snapshots to the GUI

[Tobias-Fischer]

Summary

• add a Save PDF… action to the playback GUI
• preserve GUI labels, statistics, controls, and rate history as vector PDF content
• embed only the event-camera image as pixels, preserving its nearest-neighbour appearance
• keep the exporter dependency-free and available for recordings and live streams

Checks

• cargo test -p eventcv-py --lib viewer::gui::tests::pdf_snapshot_keeps_text_vector_and_embeds_only_the_event_image
• rendered and visually inspected a PDF from data/test/sample.aedat4
• git diff --check