Add session-only TUI sidebar toggle

[alejandro-ao]

Fixes #672

Summary

• add discoverable /sidebar TUI command
• keep visibility overrides session-only while preserving configured placement
• support temporarily showing a configured-off sidebar on the default right
• cover toggle, resize, left/right, off, and persistence behavior with pilot tests
• update TUI documentation and development notes

Verification

• uv sync --dev --locked
• uv run pytest
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy
• Hugo build and Pagefind indexing
• git diff --check