Add catbox.moe / litterbox encode & upload (u)

[ekisu]

Closes #228.

Adds an encode & upload action: pressing u on the WebM maker page runs the normal encoder and then uploads the finished clip to the destination configured in the options.

upload-demo.mp4

(The demo throttles the upload so the progress meter is visible.)

What changed

• src/upload.moon (new) — the two Catbox host adapters (both use the same anonymous multipart request; litterbox adds a required time), the curl argument builder, and the clipboard/browser helpers.
• src/UploadWithProgress.moon (new) — an async curl subprocess with a live progress meter (ESC cancels), plus done and failed states.
• src/MainPage.moon — u: encode & upload next to e: encode; hidden when curl is missing.
• src/encode.moon — an optional completion callback so the upload runs after a successful encode. The detached path is skipped for uploads so completion is known.
• src/EncodeOptionsPage.moon — an Upload Destination row.
• src/options.lua — upload_host, litterbox_time, catbox_userhash, upload_curl_path, open_after_upload.
• tests/testcases/test_upload.py (new) — offline coverage using a stub curl.

Notes

• Only Catbox services are offered. Streamable's upload API requires an account (401 ... basic auth) and its web upload routes /upload and /t/upload return 404, so it is deliberately excluded.
• Uploads use curl for the multipart form. When curl is not on PATH, the u line is hidden and pressing u explains why.
• The resulting URL is written via mpv's clipboard/text when available, otherwise wl-copy / xclip / pbcopy (launched without blocking, since those tools stay alive to own the selection).
• litterbox uploads expire after litterbox_time; catbox uploads are anonymous and kept until 2 years of inactivity, or permanent/manageable when catbox_userhash is set.

Testing

nix-shell tests/default.nix --run "make && python -m tests.run_tests"
# Ran 40 tests ... OK (skipped=2)