…nfig

- Add fixed-position circular toggle button (top-right) with sun/moon SVG icons
- Add [data-theme=light] CSS variable overrides: light gray/white surfaces,
  dark slate text, same blue primary, adjusted shadows and focus rings
- Swap code block backgrounds from hardcoded rgba values to --code-bg variable
  so they adapt correctly in both themes
- Add .theme-transitioning class for smooth 300ms color transitions on toggle
- Persist theme choice to localStorage; restore on page load (defaults to dark)

- Add black and ruff as dev dependencies in pyproject.toml
- Configure [tool.black] (line-length 88, py313 target) and [tool.ruff.lint]
  (E/F/I rules, E501 ignored, E402 per-file ignore for app.py where
  warnings.filterwarnings must precede imports)
- Apply black formatting across all 13 Python source files
- Fix 29 ruff issues: unsorted imports, unused imports, duplicate StaticFiles
  import in app.py, and move stray FileResponse import to top of app.py
- Add scripts/format.sh (applies black + ruff --fix in-place)
- Add scripts/lint.sh (check-only mode for CI / pre-commit use)

- Add backend/tests/test_api_endpoints.py with 18 tests covering all three
  FastAPI endpoints (POST /api/query, GET /api/courses, DELETE /api/session/{id}).
  Tests verify status codes, response body fields, session auto-creation vs
  reuse, input validation (422 on missing query), and 500 error propagation.

- Extend backend/tests/conftest.py with shared API test infrastructure:
  _build_test_api_app() builds a minimal FastAPI app mirroring app.py routes
  with an injected rag_system, avoiding the static-file mount and ChromaDB
  init that prevent direct app.py import in tests.
  mock_rag_system fixture provides a pre-configured MagicMock.
  api_client fixture wires TestClient to the test app.

- Add [tool.pytest.ini_options] to pyproject.toml (testpaths, pythonpath,
  addopts) so `uv run pytest` works from the project root without extra args.

- Add httpx>=0.28 to dev dependencies (required by starlette TestClient
  in starlette 0.46+ / FastAPI 0.116+).

# Conflicts:
#	pyproject.toml
#	uv.lock