feat: add sandboxed App Shot window capture

[MR-Gullo]

Summary

Related to #318. Add an App Shot action that captures the frontmost window without an area-selection overlay. It shows a thumbnail, runs local Vision OCR, and writes the image followed by Markdown and plain-text context as separate clipboard states. The action is available from the capture menu or an assignable hotkey.

The workflow was inspired by ChatGPT Appshots. I also reviewed T3 Code's open-source snapshot work while exploring the design.

Testing

• Release app build succeeded.
• CONFIGURATION=Release scripts/run-tests.sh: 801 passed, 0 failed.
• Tests cover source-window selection, context output, thumbnail timing, and separate image and text clipboard states.
• Manual test on this PR's build: the assigned hotkey captured a synthetic TextEdit window, and the image remained in screenshot history after a Mac restart.
• Manual verification remains open for the OCR text and image-then-text clipboard sequence. The restart happened before those clipboard states could be inspected.
• Multi-monitor capture was not tested; only one display was active.
• The build emits compiler warnings outside the added App Shot files, so the warning-free checklist item remains open.

Notes

This change keeps App Sandbox enabled. It does not include cross-process Accessibility extraction or the passive Option-key gesture; those need a separate design decision. Nothing is uploaded or submitted automatically.

I am keeping this PR in draft until the remaining manual checks are complete.