Refactor area detector into per-dock module structure

[Osayi-ANL]

What changed

Major refactor of the area detector viewer. The monolithic area_det_viewer.py was split into a package under src/dashpva/viewer/area_det/, with each panel extracted into its own dock module (docks/image_dock.py, docks/roi_dock.py, docks/stats_dock.py, docks/mask_dock.py, docks/mouse_pos_dock.py, docks/analysis_dock.py). The CLI entry point now resolves to the new path. Subsequent commits on the branch are layered on top of this restructure:

• Initial refactor: new file structure, dock split, CLI rewired, dock header theming
• Dock header theme color + test_cli to exercise the CLI button surface
• Dock theming polish, PV channel area det switching cleanup, per-curve toggles in the stats plot,
• removal of HKL Viewer/Save buttons from the image dock in area_det
• Removed a stale HKL-clear comment block in the PV-channel switch path

Test plan

• Launch the area detector via the CLI — confirm it lands on the new area_det_viewer.py and the window opens with stats, mouse position, and mask docks initially
• Start Live View against a known PV channel — image renders, stats labels update, ROI overlays appear
• Switch the PV channel mid-session — HKL/ROI/Stats monitors rebind cleanly, no duplicate callbacks, no "Missing PV data" spam, no orphaned overlays
• Stop Live View — timers stop, no stale callbacks left attached
• Plot Stats - Toggle each stat visibility in the stats plot
• Drag/resize/rearrange docks — dock headers render with the themed color and titles stay readable in both light and dark system themes
• Confirm the image dock no longer shows the removed HKL Viewer/Save buttons
• pytest tests/unit/test_cli.py passes -- because we changed the area_det structure it passes