Add automatic FFmpeg download for standalone Windows experience

FFmpeg is now automatically downloaded on first run if not found,
eliminating the need for vcpkg or manual FFmpeg installation on Windows.

Changes:
- Add FFmpeg availability check and auto-download on startup
- Use ffprobe_path() for auto-downloaded binaries in video decoder
- Track FFmpeg availability in app state with graceful degradation
- Show warning banner when video conversion is unavailable
- Skip video files when adding to queue if FFmpeg unavailable
- Simplify build instructions in README (remove vcpkg recommendation)

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "avif-maker"
-version = "0.1.0"
+version = "0.1.1"
 edition = "2021"
 description = "Convert images and videos to AVIF format with a native GUI"
 license = "MIT"
@@ -19,6 +19,7 @@ image = "0.25"
 gif = "0.14"
 
 # Video decoding (runs ffmpeg as subprocess - no FFI issues)
+# Default features include "download_ffmpeg" for auto-downloading FFmpeg on first run
 ffmpeg-sidecar = "2"
 
 # Async & utilities