[Fazorath]

Closes #25

What

• Export CSV on the History page: one row per run with its session's details, following the current All/Abyss/Incursion filter. A session with no runs yet still gets a row.
• Back up and Restore for tracker.db, using SQLite's backup API so the copy is consistent while the app is open.
• Restore checks the file first (integrity, has sessions and runs), so a bad file leaves current data alone. Backups from older schemas are migrated on restore, and the session pages re-sync afterwards.
• Core logic is Tracker.export_csv, backup and restore, with 10 new tests in test_export.py.

Testing

• The 67 non-GUI tests pass on Python 3.13.
• The GUI tests segfault on my machine in a headless session, and they do so on untouched main too. So the new History buttons are untested by running them. Worth a click-through before merging.

Known gaps

• Restoring while a timer is running does not reset the timer or the pocket overlay.
• The CSV has no BOM, so Excel on Windows may mis-render non-ASCII text. Current values are all ASCII.