fix(ffmpeg): prefer bundled binary paths over PATH

[ubugeeei]

Finding

Electron resolves bundled ffmpeg/ffprobe paths and injects env vars, but Rust helpers may prefer PATH first. That weakens reproducibility and can accidentally use the wrong binary.

Evidence

• electron/main.ts sets FRAMESCRIPT_FFMPEG_PATH and FRAMESCRIPT_FFPROBE_PATH.
• backend/render Rust ffmpeg helpers inspect PATH-installed binaries.

Acceptance criteria

• Prefer explicit FRAMESCRIPT_* env paths before PATH.
• Validate that the selected binary exists and is executable.
• Log the selected binary path once for diagnostics.

[ubugeeei]

対応済みです。修正は PR #1 / commit 9cf4498 に反映済みです。