Copy displayed Android frame to PC clipboard

[ami3go]

What changed

• add a PNG screen-copy helper for the rendered Android canvas
• keep normal Ctrl+C text-first behavior
• fall back to copying the current Android frame as PNG when Ctrl+C produces no new Android text selection
• add Ctrl+Shift+C as an immediate forced image-copy shortcut
• keep the clipboard panel Copy button text-only
• add shortcut classification coverage

Why

Gallery and other image-viewer apps do not provide image data through scrcpy's text clipboard path. The existing Ctrl+C write-through could therefore reuse stale Android clipboard text. This change uses the already-rendered browser canvas as the image source instead.

Validation

• reviewed the branch diff against current main
• added Node test coverage for smart vs forced-image shortcut classification
• local npm build/test execution could not be run in this environment because the runtime cannot access GitHub to clone dependencies and GitHub CLI is not installed