dsh-fullscreen-settings: Turning the DSH Settings Dialog into a Codex-Style Full-Page Settings Page

The `dsh-fullscreen-settings` plugin refactors DeepSeek Harness (DSH)'s default centered settings dialog into a full-screen page. It removes the background overlay, fills the window, and adopts a Codex-like left-right split-pane layout to improve the reading experience for long lists. The plugin overrides default rendering through the Cordis Slot mechanism and anchors styles using element selectors without modifying the official source code. It supports system-level macOS adaptations, is compatible with DSH 0.2.0-rc.1, and provides a safe rollback mechanism.