cli: drop stale "+ ffmpeg" from --help about string

The Wayland path moved from a shelled-out ffmpeg to an in-process
GStreamer pipeline back in the 2026-05-16/18 pivot, but the clap
`about` string still advertised ffmpeg. Now reads "P2P screen sharing
over iroh".

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/cli.rs

@@ -4,7 +4,7 @@ use clap::{Parser, ValueEnum};
 #[command(
     name = "pixelpass",
     version,
-    about = "P2P screen sharing over iroh + ffmpeg",
+    about = "P2P screen sharing over iroh",
     long_about = "Run with no arguments for an interactive Host/View menu. \
                   Pass a ticket positionally to skip the menu and view headlessly."
 )]