Surface the 20 MB CSV upload cap in the upload UI

[jeanclawd]

Problem

CSV uploads larger than ~20 MB fail because of Pyodide's WASM heap limit (documented in `CLAUDE.md`). Users only discover the cap by trying — the upload silently errors out (or surfaces a cryptic Python exception) and they assume the app is broken.

Proposal

Show the cap before the upload starts:

• In the upload modal (`frontend/index.html`, the CSV upload section): a small badge or note "Max 20 MB · runs in your browser".
• Client-side check: if `file.size > 20 * 1024 * 1024`, reject with a friendly error toast ("File is too large for the browser runtime — please subsample first") before calling `pyCallBinary`.

Acceptance criteria

• The 20 MB limit is visible in the UI before the user picks a file.
• An oversized file produces a clear, user-facing error and never reaches Pyodide.
• Files within the limit upload as today.

Why now

Two lines of HTML, ~10 lines of JS. Removes the worst silent failure mode in the app.