Added black as an automatic code formatter and created a development script for running quality checks.

• Added black>=25.1.0 to the dev dependency group
• Added [tool.black] configuration section with line-length = 88 and target-version = ["py313"]

• Created a development script for running code quality checks
• ./quality.sh — checks formatting without making changes (useful for CI)
• ./quality.sh format — auto-formats code with black

All Python files in backend/ and main.py were formatted with black for consistency:

• backend/ai_generator.py
• backend/app.py
• backend/config.py
• backend/document_processor.py
• backend/models.py
• backend/rag_system.py
• backend/search_tools.py
• backend/session_manager.py
• backend/vector_store.py
• backend/tests/test_ai_generator.py
• backend/tests/test_course_search_tool.py
• backend/tests/test_rag_system.py

Formatting changes were minimal — mostly trailing whitespace cleanup, consistent blank lines between classes/functions, and alignment comment spacing to match black's style.

No frontend files were modified. All changes were to the backend testing infrastructure.

backend/tests/conftest.py — Shared pytest fixtures:

• mock_rag_system — Creates a RAGSystem with VectorStore, AIGenerator, and DocumentProcessor all mocked out
• mock_vector_store — Standalone mock VectorStore for tool-level tests
• sample_query_payload / sample_query_with_session — Reusable request payloads for API tests

backend/tests/test_api.py — 16 API endpoint tests across 3 test classes:

• TestQueryEndpoint (8 tests) — POST /api/query: success response, session creation/reuse, validation errors, 500 on RAG failure, response shape and source structure
• TestCoursesEndpoint (4 tests) — GET /api/courses: success response, response shape, 500 on error, empty catalog
• TestMiscRoutes (4 tests) — 404 on unknown routes, 405 on wrong HTTP methods, 422 on malformed JSON

The test app is built inline via create_test_app() to avoid importing backend/app.py which mounts static files from ../frontend (unavailable in the test environment).

pyproject.toml — Added:

• httpx dev dependency (required by FastAPI's TestClient)
• [tool.pytest.ini_options] section with testpaths, pythonpath, and addopts for cleaner test execution (uv run pytest from repo root now works without extra flags)

• Added data-theme="dark" attribute to <body> for CSS theme switching
• Added a fixed-position theme toggle button before the main container with:
  • Sun icon (shown in dark mode, click to switch to light)
  • Moon icon (shown in light mode, click to switch to dark)
  • aria-label for screen reader accessibility
• Bumped CSS cache version to v=11

• Light theme CSS variables: Added [data-theme="light"] block with light background/surface colors, dark text, adjusted borders and shadows for good contrast
• Added --code-bg variable to both themes so inline code and pre blocks adapt to the theme (replaces hardcoded rgba(0,0,0,0.2))
• Theme toggle button styles: Fixed position top-right, circular 44px button, hover scale animation, focus ring for keyboard navigation
• Icon transition styles: Sun/moon icons crossfade with rotation using CSS opacity and transform transitions
• Global theme transition: Added transition on background-color, color, border-color, and box-shadow for smooth theme switching
• Source link colors: Changed from hardcoded blue (#5ebbf7) to var(--primary-color) so links work in both themes

• initTheme(): Reads saved theme from localStorage on page load and applies it to data-theme
• toggleTheme(): Switches between dark/light, saves preference to localStorage
• updateThemeAriaLabel(): Updates the toggle button's aria-label to reflect the available action
• Registered click listener for the theme toggle button in setupEventListeners()

• Default theme is dark (matching the existing design)
• Theme preference persists across page reloads via localStorage
• The toggle button uses a 44px touch target for mobile accessibility
• Sun icon = "click to get light", Moon icon = "click to get dark"
• All transitions are 300ms ease for a smooth, non-jarring switch