Harden media operations and add isolated clients

[damianociarla]

Summary

• own concurrent FFmpeg/ffprobe initialization, cancel siblings, and preserve the root error
• reject option-like output paths and unsafe filter colors across every output API
• calculate progress against effective trim duration
• add isolated reusable createClient() contexts with fixed process configuration
• clarify the supported input model and trusted custom argument APIs
• improve landing-page progressive enhancement, mobile rendering, and no-JS 404 behavior
• aggregate all quality, unit, and real-FFmpeg matrices behind CI / Required

Verification

• npm run check
• npm run test:integration with FFmpeg 9.0.1
• 237 unit tests passing, 2 skipped integration tests in the unit run
• 98.59% statement / 99.51% line coverage
• desktop and 390x844 browser QA, including 404 hydration and console checks
• npm audit --omit=dev (0 vulnerabilities)