Tasks: Docked Changes panel

• T001 Turn changes.rs into a persistent ChangesPanel with show/clear/focus
• T002 Group rows by directory, file-name titles, +N −N suffix in diff colours
• T003 Dock the panel in an end-side AdwOverlaySplitView; header toggle and Ctrl+Shift+D
• T004 Breakpoints: overlay below 1100sp, keep the 760sp narrow layout winning, hide the toggle at 360px
• T005 Mirror sidebar colours into --secondary-sidebar-*
• T006 Port the changes and file-reader GTK tests; capture light/dark/narrow