🪲 BUG-#36: Fix sidebar chevron color and submenu alignment

[FernandoCelmer]

Closes #36

Summary

• Recolor the collapse/expand chevron using a CSS mask instead of a hardcoded black/white SVG, so it now follows the Dracula palette: purple at rest, cyan on hover, pink when expanded
• Replace percentage-based submenu indentation (margin-left: 10%) with a fixed .75rem indent plus a left border guide, so nested items visually belong to their parent
• Auto-expand the sidebar section containing the currently active page (menu.html / dropdown-menu.html), instead of requiring a manual click every time you land on a submenu page
• Add tests/test_sidebar_menu.py: builds a real nested mkdocs site and asserts the active section renders collapse show / aria-expanded="true", while unrelated sections stay collapsed
• Add tests/test_sidebar_style.py: asserts the chevron references Dracula palette variables (no hardcoded stroke colors) and the submenu indent has no percentage-based margins
• Update screenshot.png (recompressed, same image)

Test plan

• pytest tests/ passes (49/49)
• ruff check / ruff format --check pass
• Verified via mkdocs build + headless Chrome screenshots: chevron colors, submenu indent, and auto-expand on /nav/development/development-guide/ (Development expanded, Settings stays collapsed)