SnapShots: capture a selected screen region (macOS)

[pavan-tx9]

Problem

SnapShots always capture the whole focused window. Often I only want part of the screen, like one panel, an error toast, a chart, or an area that spans two windows. Today that means capturing the full window and letting the agent work through the noise, or switching to the OS screenshot tool and pasting the result in.

Proposal (smallest version)

Add a Capture area setting to Settings → SnapShots: Window (default, unchanged) or Region.

• In Region mode the existing capture shortcut opens macOS's native drag-to-select tool (screencapture -i -s). It uses the Screen Recording permission SnapShots already asks for. The selected area attaches to the draft through the same pipeline as window captures.
• Esc cancels quietly, with no "Snapshot failed" toast.
• macOS only to start, and hidden elsewhere. Windows already has an arbitrary-rectangle capture worker but would need a selection overlay. Linux's screenshot portal has an interactive area mode.
• Region captures carry no app text, flash, or fly-in animation, because there is no single window to attribute them to.

Prototype

Draft PR with tests and screenshots: #27786

Questions for maintainers

1. Is region capture something you want in SnapShots?
2. Is a mode setting on the existing shortcut the right shape? The alternative is a separate, second region shortcut, so both are available at once. That needs a second shortcut recorder and registration path, which is a noticeably larger change.
3. Is macOS-only acceptable as a first step?

[pavan-tx9]

Duplicate of #27784 (the converted feature request). I posted this proposal there instead, so please reply on #27784.