Add markup layers and tools to screenshot editor

[multiplex55]

Motivation

• Provide a proper markup model (strokes, rectangles, arrows, highlights) separated from the base image so annotations can be composed, edited, and rendered reliably.
• Enable freehand pen drawing with an undo/redo workflow and expose common toolbar controls for selecting tools, colors, and thickness.
• Allow launching the editor already in a markup mode from screenshot commands (e.g. region markup) and ensure markup is applied before cropping when saving/copying.

Description

• Introduced a MarkupLayer model and supporting types (MarkupStroke, MarkupRect, MarkupArrow, MarkupHistory) and rendering helpers in src/gui/screenshot_editor.rs, including render_markup_layers and simple raster primitives (draw_line, draw_circle, blending).
• Reworked ScreenshotEditor to store history: MarkupHistory and transient state for active strokes/shape creation, added tool, color_index, and thickness fields, and updated apply_edits() to render markup layers first then apply cropping (src/gui/screenshot_editor.rs).
• Added an in-editor toolbar for tool selection (pen/arrow/rect/highlight), a 5-color palette, thickness controls (buttons and [/] key handlers) and undo/redo buttons and shortcuts in the editor UI (src/gui/screenshot_editor.rs).
• Added new screenshot action/command screenshot:region_markup / shot region markup that auto-opens the editor in pen mode and threaded the MarkupTool through launch_editor and open_screenshot_editor (src/plugins/screenshot.rs, src/gui/mod.rs).
• Exported new types/functions from GUI module (render_markup_layers, Markup* types) and added unit tests exercising rendering and undo/redo behavior (tests/screenshot_plugin.rs).

Testing

• Ran formatting with cargo fmt which completed successfully.
• Ran cargo test --test screenshot_plugin to build and run the unit tests added in tests/screenshot_plugin.rs (which include markup_layers_render_on_image and markup_history_undo_redo), but the test run failed during build due to a missing system library when compiling a dependency: alsa-sys reported a missing alsa.pc (pkg-config / ALSA system library), causing the test invocation to abort; tests themselves were added and exercise pixel-level changes and history undo/redo semantics.

---

Codex Task