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)

README.md

@@ -20,45 +20,51 @@ A native GUI application to convert images and videos to AVIF format.
 | PNG/APNG     | `image` crate       | Full          | Static images              |
 | JPEG         | `image` crate       | N/A           | No alpha                   |
 | WebP         | `image` crate       | Full          | Static only                |
-| MP4/MOV/WebM | `ffmpeg-sidecar`    | Codec-dependent | Requires ffmpeg in PATH     |
+| MP4/MOV/WebM | `ffmpeg-sidecar`    | Codec-dependent | FFmpeg auto-downloaded on first run |
 
 ## Build Requirements
 
 ### macOS
 
 ```bash
-# Install dependencies
-brew install libavif ffmpeg
+# Install build dependencies (libavif is built from source)
+brew install cmake nasm
 
 # Build and run
 cargo build --release
 ./target/release/avif-maker
 ```
 
+FFmpeg is automatically downloaded on first run if not found in PATH.
+
 ### Linux (Ubuntu/Debian)
 
 ```bash
-# Install dependencies
-sudo apt install libavif-dev ffmpeg
+# Install build dependencies (libavif is built from source)
+sudo apt install build-essential cmake nasm
 
 # Build and run
 cargo build --release
 ./target/release/avif-maker
 ```
 
+FFmpeg is automatically downloaded on first run if not found in PATH.
+
 ### Windows
 
-Use vcpkg or pre-built binaries for libavif and FFmpeg.
+The application automatically downloads FFmpeg on first run if not found. No manual setup required for end users.
+
+To build from source, install:
+- [Visual Studio Build Tools](https://visualstudio.microsoft.com/visual-cpp-build-tools/) (with C++ workload)
+- [CMake](https://cmake.org/download/)
+- [NASM](https://www.nasm.us/) (add to PATH)
 
 ```powershell
-# With vcpkg
-vcpkg install libavif ffmpeg
-
-# Set environment variables
-$env:VCPKG_ROOT = "C:\path\to\vcpkg"
-
 # Build
 cargo build --release
+
+# Run
+.\target\release\avif-maker.exe
 ```
 
 ## Usage