fix(desktop): Windows FFmpeg from BtbN GitHub + detect bin/ layout (#248)

[thcp]

Fixes #248.

Problem

Two issues reported for Windows FFmpeg setup:

1. Slow download. First-run setup fetches FFmpeg from gyan.dev, a single US-hosted mirror. A German reporter saw ~0.1 MB/s (vs 10 MB/s pulling FFmpeg from GitHub on the same connection).
2. Manual FFmpeg not detected. The reporter placed FFmpeg at data/ffmpeg/bin/ffmpeg.exe (the upstream folder layout), but detection only checked data/ffmpeg/ffmpeg.exe, so it downloaded anyway.

Changes (all in desktop/src-tauri/src/main.rs)

1. Windows FFmpeg download: gyan.dev -> BtbN GitHub

• Default asset is now ffmpeg-n8.1-latest-win64-gpl-8.1.zip from BtbN/FFmpeg-Builds, served via GitHub's CDN (fast worldwide, including the EU).
• Checksum verification reworked: BtbN publishes one combined checksums.sha256 (every asset as <hash> <filename> lines) instead of gyan's per-file {url}.sha256. A new pure helper sha256_from_checksums extracts the line for our archive. Fails closed if the archive is not listed.
• A custom STEMDECK_FFMPEG_URL override still skips verification (unchanged behavior).
• Extraction is unchanged: it already flattens by matching ffmpeg.exe/ffprobe.exe anywhere in the archive, so BtbN's bin/ layout and the extra ffplay.exe are handled.

GPL is fine here because StemDeck shells out to FFmpeg as a separate process (mere aggregation), not linking it.

2. Detect a manually-placed FFmpeg under data/ffmpeg/bin/

• New resolve_existing_ffmpeg checks the STEMDECK_FFMPEG override, then flat data/ffmpeg/, then data/ffmpeg/bin/, returning the first that exists.
• Wired into probe_runtime, ffmpeg_dir_if_present, and ensure_ffmpeg, so a manually-placed build is honored instead of triggering a download.
• write_setup_config now records ffprobe next to the resolved ffmpeg.

Scope

Windows only (macOS/evermeet and Linux/johnvansickle sources unchanged). The bin/ fallback is cross-platform since it is harmless elsewhere.

Testing

• cargo fmt --check: clean.
• cargo test: 11 passed, including 2 new tests (sha256_from_checksums_picks_matching_line, resolve_existing_ffmpeg_prefers_flat_then_bin).
• The Windows download path is #[cfg(windows)] and could not be built on the macOS dev box (cross-compile fails on the ring C toolchain); it builds in the Windows release job. Manual Windows checks still pending: real first-run download from BtbN, and bin/ detection without a download.