feat(editor): add visual adjustments and fades

[sjungwon03]

Summary

• Add per-visual-clip brightness, contrast, saturation, opacity, and fade-in/out controls in the web editor inspector.
• Composite overlapping visual layers in insertion order for preview and local WebM export; the same pure model computes opacity, filter, and effective audio gain.
• Preserve outer fades across clip splits and reduce fades when trimmed, and keep the inspector available at narrower widths.
• Update the fork README to describe the supported basic adjustments and remaining Classic catalog limits.

Verification

• apps/web TypeScript check passed.
• apps/web Vitest 13/13 passed, including overlap/trim rules and mocked layered browser export.
• apps/web Vite production build passed.
• No browser executable is attached for a manual rendered-media check; CI and reviewer should inspect preview/export parity.

Integration

This PR targets Theorvane/OpenCut main only. After review and merge, OpenScene issue Theorvane/OpenScene#464 will advance the pinned submodule in a separate PR. No upstream OpenCut PR is planned.

[sjungwon03-ai]

Review summary

Verdict: Approved

• Reviewed: visual-layer compositing, adjustment and fade model, browser preview, WebM export, and trim/split behavior.
• Checks: All four GitHub Actions checks passed; local npm test (13/13), npm run build, and tsc --noEmit passed.