feat(ui): Sessions screen shows the real span of a session tracked in intervals

[mbel-tech]

What and why

The dropped SP5 item 5, done at the Sessions screen instead of in the reader. A file with fnum 0-199 and 400-599 showed 400 frames and 13 s against a real 20 s.

• SessionFacts gains span_frames (end of the last tracking interval, only when it exceeds the rows read) plus video_frames and frames_text.
• The Frames column and the detail panel read "400 of 600" for such a session; the minutes in the detail panel and the summary line use the real span.
• Sessions without gaps look as before. video.n_frames stays the row count, so the reader contract is untouched.

Test plan

Written first (failed on the missing attribute): span from the last interval, none without gaps or with one interval from 0, table and detail text for gappy and gap-free sessions.

• pytest tests/test_ui: 920 passed (Qt offscreen)
• ruff check passes

Checklist

• Tests added or updated (written before the fix)
• ruff check passes
• CHANGELOG.md entry under [Unreleased]
• Guide screenshots: the guide's Sessions screenshots use gap-free sessions, so none change

🤖 Generated with Claude Code

https://claude.ai/code/session_012btUjvTQTf6i51Rpr3t8Hc

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