fix: retain scroll position in details panel tables during periodic updates

[stabldev]

Summary

Fixes the issue where scrolling down tables in the torrent details panel (Peers, Trackers, Files) would jump back to the top whenever the 1-second polling update occurred.

Cause

Previously, update_peers(), update_trackers(), and update_files() called table.clear() on every tick, which emptied all rows and reset Textual's viewport scroll_y offset to 0.

Fix

• In-place row updates (Key Diffing): Assigned unique row keys (peer.ip, tracker.url, file.index).
• On each tick:
  • Existing rows are updated in-place via table.update_cell(...) only when cell contents change, leaving the table's DOM and scroll offset untouched.
  • New entries are appended via table.add_row(..., key=key).
  • Disconnected entries are removed via table.remove_row(...).
• Added clear_tables() to DetailsPanel called when selecting a different torrent to ensure data refreshes cleanly between torrents.
• Added comprehensive unit tests in tests/test_widgets_details_panel.py verifying scroll position retention and in-place row updates.