fix(screenshot): capture display containing Thuki window on multi-monitor setups

[quiet-node]

Overview

/screen previously always captured the primary (menu-bar) display. On a multi-monitor setup, users invoking /screen while Thuki is shown on a secondary display would receive a screenshot of the wrong monitor. This PR makes /screen capture the display Thuki's own window is currently shown on.

How it works

1. Before dispatching the screenshot pipeline, the Tauri command resolves Thuki's WebviewWindow and asks AppKit ([NSScreen deviceDescription][NSScreenNumber]) for the CGDirectDisplayID of the screen the window lives on. This is the canonical multi-monitor query and sidesteps the coordinate-conversion mismatches that occur on mixed-DPI setups (e.g. a 2x retina primary + 1x secondary).
2. The display ID is converted to its Quartz-coordinate rectangle via CGDisplayBounds and reduced to a center anchor point.
3. capture_full_screen_raw accepts an Option<(f64, f64)> anchor. The anchor is hit-tested with CGGetDisplaysWithPoint; the matching display's bounds are passed to CGWindowListCreateImage.
4. Fallback chain when any step fails (window missing, window offscreen, point outside all displays): defaults to the main (menu-bar) display, preserving prior behavior.

The CoreGraphics display helpers that previously lived inline inside lib.rs are extracted into src-tauri/src/cg_displays.rs, with one new function bounds_for_display(display_id) for ID-based lookup. The activator continues to use the existing point-based helpers without change.

Highlights

• New module src-tauri/src/cg_displays.rs consolidates CGGetDisplaysWithPoint, CGDisplayBounds, CGMainDisplayID FFI behind three small wrappers.
• nswindow_display_id performs raw Objective-C messaging (screen → deviceDescription → NSScreenNumber) without pulling in extra objc2-app-kit features.
• display_bounds_center is a pure helper, unit-tested for primary, offset, negative-origin, and zero-size cases.
• Strict main-thread discipline: the NSWindow pointer is fetched off-thread but only dereferenced inside run_on_main_thread.
• Backwards-compatible API: capture_full_screen_pixels takes Option<(f64, f64)>; None reproduces prior single-display behavior.

Testing

• bun run test:all:coverage — frontend + backend tests pass; backend cargo llvm-cov gate holds at 100% line and function coverage. CoreGraphics and Objective-C FFI wrappers are marked #[cfg_attr(coverage_nightly, coverage(off))] because they require a live window server and a real NSWindow to exercise; their consumers exercise the surrounding logic.
• bun run validate-build — lint, format, typecheck, and release build complete with zero warnings/errors.
• Manual: invoked /screen on a multi-monitor setup with Thuki shown on the secondary display; resulting screenshot is the secondary display, not the primary. Confirmed fallback to primary when Thuki is hidden.

Note

The Objective-C messaging in nswindow_display_id relies on the autorelease pool maintained by the main-thread runloop. This is the standard contract for AppKit calls dispatched via run_on_main_thread and is consistent with the existing screenshot pipeline's use of CGWindowListCreateImage.