feat: AR Display Manager — multi-monitor floating app switcher

Adds the AR Display Manager daemon (Task #9):

- display_manager/ package with 8 modules:
  - app_registry.py   : static metadata for the 4 bridge apps
  - config.py         : JSON-persisted config + per-screen layout store
  - win32_utils.py    : ctypes wrappers (EnumWindows, SetWindowPos, ShowWindow)
  - process_manager.py: launch + track app processes, HWND lookup
  - floating_button.py: always-on-top 52×52 px AR button per monitor
                        with draggable placement + custom-painted popup
  - display_manager.py: orchestrator (QScreens → buttons → app placement
                        + system tray with Settings/Launch/Quit)
  - settings_dialog.py: modal dialog for exe paths, button position,
                        and Windows autostart toggle
  - autostart.py      : HKCU Run registry read/write helpers
- display_manager_main.py at repo root: launcher script
- Studio Overview tab: "Lanzar Display Manager" button
- pyproject.toml: display-manager optional dependency group (PySide6)

Layout persistence: ~/.ar-autopilot/display_manager/layout.json
Config:            ~/.ar-autopilot/display_manager/config.json
Supports up to 4 monitors (2 HDMI native + 2 USB DisplayLink).
Double-click tray icon to toggle button visibility.

AR_electronics — AR-Autopilot Project

display_manager_main.py

@@ -0,0 +1,25 @@
+"""AR Display Manager — repo-root launcher.
+
+Usage:
+    python display_manager_main.py
+    python -m display_manager.app
+
+The daemon detects all connected monitors and places a small floating
+AR button in the corner of each.  Clicking the button opens the app
+switcher: choose which of the four AR Bridge apps you want visible on
+that monitor.  The chosen app's window is moved and maximized there;
+the others continue running in the background.
+
+Layout (which app is on which screen) is persisted in
+~/.ar-autopilot/display_manager/layout.json so the arrangement
+survives restarts.
+"""
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parent))
+
+from display_manager.app import run
+
+if __name__ == "__main__":
+    sys.exit(run())