docs: improve README feature tours

[bifrost0x]

What changed

• replaced all three README GIFs with longer, readable 1280×800 tours
• expanded the demos to cover multi-session layouts, broadcast input, SFTP editing, command search, user commands, sudo-enabled command sets, reordering, and profile assignment
• switched the GIF references to relative paths and improved their alt text

Why

The previous animations were too small and skipped important parts of the current workflows. These replacements provide a clearer first impression of the recent features.

Validation

• pytest tests -q — 266 passed, 28 skipped
• pytest tests/test_command_set_ui.py -q — 13 passed
• node --test tests/js/command-set-utils.test.js — 5 passed
• all three GIFs decoded successfully, loop continuously, and meet the planned duration ranges