feat: show upload progress and skip re-uploads via checksum dedup

[a-essawy]

What

Two upload UX improvements for rb ffmpeg with local inputs:

1. Checksum dedup: the CLI now computes the file sha256 and passes it to uploads.create(). The API already dedups on (org, checksum): re-running with the same input file skips the byte transfer entirely and reuses the existing ready asset — repeat uploads drop from seconds (or minutes for large files) to one ~300ms round trip.
2. Upload progress in the spinner: wires the SDK onProgress callback into the step spinner. The Uploading line now shows filename · 4.2 MB / 12.0 MB (and 1/2 prefixes for multiple inputs) instead of a mute timer. Single-PUT uploads report on completion; multipart uploads report per part.

Tests

• sha256 checksum passed to the SDK (hash computed independently in the test)
• progress events forwarded with per-file context
• 33/33 tests in touched files green; typecheck clean
• main-welcome.test.ts failures on Windows dev machines are pre-existing (child-spawn of bun PATH issue, passes in CI)

Notes

Unrelated but found while testing: package.json version on main is 1.0.0 (manifest repair in #50 fixed .release-please-manifest.json but not package.json), so built binaries self-report v1.0.0 and the update check misfires. Needs its own state-repair PR.