Clean window capture with drop shadow #114

Summary

Capture a specific application window isolated on a white/transparent background with a professional drop shadow — no desktop clutter or overlapping windows bleeding in.

Why

This is CleanShot X's signature feature on Mac and is largely unaddressed by free Windows tools (see docs/feature-research.md, Tier 1 quick win). High perceived polish for low effort.

Scope

• Detect the window under the cursor and capture just that window.
• Composite onto a configurable background with padding + drop shadow.
• Reuse the existing beautifier render pass where possible.

Acceptance criteria

• User can pick a window and capture it isolated.
• Output has a clean background, padding, and drop shadow.
• Result opens in the annotation overlay.

Entry points

BeautifierRenderService (existing background/padding/shadow pass), ScreenCapture.