Add web frontend

[gaurav]

This PR adds a web interface to babel-explorer, exposing all four tools — NodeNorm, XRefs, IDs, and Test Concordance — through a browser UI, a JSON REST API, and CSV downloads. It also includes the full CLI implementation, core query engine, and comprehensive test suite that the web frontend builds on.

Why FastAPI?

We considered several options for the web framework:

• Streamlit — Easy to prototype with, but its re-run-the-whole-script model doesn't map well to babel-explorer's architecture (shared BabelDownloader/BabelXRefs instances with LRU caches, lazy multi-GB file downloads). It also wouldn't give us a clean REST API for programmatic access.
• Flask — A solid choice, but we'd need to bolt on separate API documentation (Swagger/OpenAPI) and pick additional libraries for request validation.
• Django — Too heavyweight for a tool with no database models or user auth.
• FastAPI was the best fit because:
  • It auto-generates Swagger docs at /docs, giving us a REST API with interactive documentation for free.
  • It runs synchronous route handlers in a threadpool automatically, which is exactly what we need — the core code uses synchronous requests and DuckDB, and some queries trigger multi-GB Parquet downloads that would block an async event loop.
  • Jinja2 templating is a first-class integration, so we get server-rendered HTML without a JS build system.
  • Combined with htmx, forms submit via AJAX and swap in HTML fragments without writing any frontend JavaScript framework code.

What's included

• Web app (src/babel_explorer/web/) — FastAPI app factory, routes, Jinja2 templates with Bootstrap 5 + htmx
• Four tool pages with forms that return results as HTML tables via htmx
• JSON REST API (/api/nodenorm, /api/xrefs, /api/ids, /api/test-concord) with query parameter interface
• CSV downloads for all tools (/api/*/csv)
• NodeNorm instance dropdown populated from NodeNorm.URLs with a "Custom URL..." option
• CLI command babel-explorer web with --host, --port, --reload options
• Navbar with links to all tools, Swagger docs, and GitHub repo
• 25 unit tests (tests/test_web.py) covering HTML pages, htmx partials, JSON API, CSV endpoints, and helper functions — all mocked, no network needed
• Core modules: BabelDownloader, BabelXRefs, NodeNorm, Click CLI, and 80+ tests across test_downloader.py, test_babel_xrefs.py, test_nodenorm.py

How to test

uv sync --group dev
uv run babel-explorer web          # http://127.0.0.1:8000
uv run pytest tests/test_web.py -v # 25 tests, ~0.4s