Feature request: SnapShots region capture (drag to select an area) with its own shortcut

[pavan-tx9]

Summary

SnapShots can only capture a whole window (the focused one). You can't drag out a region of the screen and drop that into the draft. Most of the time I only want part of a window, like one panel, an error toast, or a chart. Sometimes I want an area that spans several windows. Today I have to capture the full window or fall back to the OS screenshot tool and paste the result in.

Current behavior

• Settings → SnapShots has a single Capture shortcut, and it always captures the active window (docs/user/snap-shot.md: "A SnapShot captures the window you are working in").
• On macOS, captureMacWindowSnapshot captures the active window via /usr/sbin/screencapture. On Windows, captureRegionWindowSnapshot / RegionSnapShotWorker crops to the active window's bounds. Despite the name, there is no user-selected region.
• The Settings UI has no region or area option and no second shortcut.

Proposed

Add a Capture region mode with its own shortcut, separate from the window shortcut, for example:

Setting	Default
Capture shortcut (window)	existing
Capture region shortcut	unset, or e.g. ⌃⇧R

Pressing it shows a crosshair/drag selection overlay. The selected rectangle is attached to the current draft the same way window captures are, with the same sound, flash, and fly-in animation, and the same pending-capture persistence.

Implementation notes:

• macOS: screencapture -i -s (or -i -R) gives native interactive selection and reuses the Screen Recording permission SnapShots already asks for.
• Windows: the existing xa11y screenshot({ region }) worker already takes an arbitrary rectangle. It just needs a selection overlay to supply one.
• Linux/Wayland: the screenshot portal's interactive mode (already used as the "Manual capture only" fallback) supports area selection.
• Accessibility data: either include the elements that intersect the region from the window underneath, or skip it for region captures (same as picker captures today).

Environment

T3 Code desktop, macOS. Checked against main @ bf270e74.

[pavan-tx9]

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?