fix(cli): fail video snapshots without FFmpeg

[miguel-heygen]

What

Video-backed snapshots now fail with an actionable FFmpeg installation hint instead of succeeding with a black or blank video region when FFmpeg is unavailable.

Why

snapshot previously skipped frame extraction when it could not resolve FFmpeg, then silently fell through to Chrome's unreliable video seek path. On Windows environments without system FFmpeg this produced misleading black snapshots even though the command exited successfully.

How

The video-frame extraction path now requires the resolved FFmpeg executable and surfaces the existing platform-specific installation guidance when it is missing. DOM-only snapshots are unchanged because the check runs only for active video extraction.

Test plan

• Unit tests added/updated: bunx vitest run packages/cli/src/commands/snapshot.test.ts (14 passed)
• Manual testing performed: reproduced black MP4 snapshot on 0.7.54 without FFmpeg; patched CLI exits 1 with an actionable install command
• Full monorepo build: bun run build
• CLI typecheck and targeted format check
• Documentation updated (not applicable)

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[miguel-heygen]

Consolidated with the related snapshot and frame diagnostics fixes in #2402, which preserves this PR’s behavior and regression coverage.