The "FFmpeg not found" hint was hardcoded to `sudo apt install ffmpeg`
for any non-macOS platform — Windows users would see an apt command that
doesn't exist on their system, and Red Hat / Arch users got the wrong
package manager too.
`getFFmpegInstallHint()` already exists in browser/ffmpeg.ts (and is
already used by render.ts) and handles darwin / linux / win32 correctly.
Use it here too.
Also rewrite checkFFprobe:
- it previously used `which ffprobe` which is not available on Windows
(cmd uses `where`), so on Windows the check always reported "Not
found" even when ffprobe was on PATH
- run `ffprobe -version` directly instead, which works cross-platform
whenever ffprobe is resolvable on PATH, and surfaces the version
string in the same style as the FFmpeg check
