feat: bundle yt-dlp and ffmpeg as sidecars

The app is now self-contained: yt-dlp, ffmpeg and ffprobe ship inside
the bundle and are resolved beside the executable, so a fresh Mac needs
nothing installed. A system copy still wins when present, which is how
to run a newer yt-dlp than the bundled one. yt-dlp is pointed at the
bundled ffmpeg explicitly, since a bundled app has no reason to have one
on PATH.

The binaries stay out of git (~140MB); scripts/fetch-binaries.sh pulls
them. Homebrew's ffmpeg cannot be used — it links a dozen dylibs and
does not relocate — so the static arm64 build is used instead.

Version checks moved to the async Command. yt-dlp is a PyInstaller
bundle that unpacks ~37MB on first run, and the blocking call was
stalling a runtime worker for roughly twenty seconds.

Bundle size goes from 19MB to 153MB.

README.md

@@ -4,7 +4,7 @@ A desktop app that merges all your YouTube subscriptions into one newest-first f
 downloads the videos you pick, and — when you have no connection — shows only what you
 already downloaded. Built for the flight.
 
-Tauri 2 · Rust · React · Tailwind CSS 4 · SQLite
+Tauri 2 · Rust · React · Tailwind CSS 4 · SQLite · self-contained (bundles yt-dlp + ffmpeg)
 
 Styled to `DESIGN-SYSTEM.md`: slate and sky, a 9–15px type ladder, outline-first
 controls, borders for separation and shadows only for elevation, with light and dark
@@ -39,13 +39,20 @@ deliberate: every download is guaranteed to play inside FlightTube.
 
 ## Requirements
 
+None at runtime — `yt-dlp`, `ffmpeg` and `ffprobe` ship inside the app as Tauri sidecars,
+so a fresh Mac needs nothing installed. Settings shows each tool's version and whether it
+came from the bundle or the system; a copy on your machine takes precedence, which is how
+you run a newer yt-dlp than the bundled one.
+
+The binaries are not in git (~140 MB together). Fetch them once before building:
+
 ```bash
-brew install yt-dlp ffmpeg
+./scripts/fetch-binaries.sh
 ```
 
-Both are checked at startup; Settings shows their versions and this command if either is
-missing. `ffmpeg` is required because best-quality H.264 and AAC arrive as separate
-streams that must be merged.
+That pulls yt-dlp from its official GitHub release and static arm64 ffmpeg/ffprobe from
+osxexperts.net — Homebrew's ffmpeg links a dozen dylibs and cannot be relocated into an
+app bundle. Note that static ffmpeg builds are GPL, which matters if you redistribute.
 
 ## Running it
 
@@ -78,7 +85,9 @@ system by default; Settings offers System / Light / Dark.
 - The Atom feed returns only the **~15 most recent videos per channel**. There is no
   backfill and no pagination — this is a rolling recent window, not an archive.
 - Quality tops out at 1080p, by the deliberate choice described above.
-- `yt-dlp` needs occasional updating (`brew upgrade yt-dlp`) as YouTube changes.
+- The bundled `yt-dlp` is frozen at build time and YouTube changes often. Re-run
+  `./scripts/fetch-binaries.sh` and rebuild, or install a newer one on your system —
+  the app prefers a system copy when it finds one.
 - Downloading videos is contrary to YouTube's Terms of Service.
 
 ## Tests