docs(linux): mention yt-dlp as an optional prerequisite (#973) (#997)

The preflight system check already warns in-app when yt-dlp is missing
(Voice Gallery/Dub YouTube downloads fail without it), but the install
docs never mentioned it — a user has to hit the in-app warning first
instead of seeing it up front alongside the other optional prereqs.

Co-authored-by: mergetest <test@local>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

docs/install/linux.md

@@ -12,6 +12,10 @@ working OmniVoice Studio install on a Debian / Ubuntu / Fedora / Arch host.
 - **~10 GB free disk** for the app, its Python environment, and model weights.
 - Optional: an **NVIDIA driver** for CUDA GPU acceleration — the app runs
   CPU-only without one. For AMD GPUs see [AMD GPU (ROCm)](#amd-gpu-rocm).
+- Optional: **yt-dlp** for downloading YouTube/video clips directly in the
+  Voice Gallery and Dub tabs — `sudo apt install yt-dlp` (Debian/Ubuntu),
+  `sudo dnf install yt-dlp` (Fedora), or `sudo pacman -S yt-dlp` (Arch).
+  Without it those downloads fail; everything else works fine.
 
 That's it — Python, FFmpeg, and the model weights are bundled or bootstrapped
 by the app itself on first launch. No toolchain needed.