- replace old PopupMenu (flat preset list) with PopupPanel-based menu
- add 3-icon toolbar: ⚙ Settings, ℹ Info, ✕ Remove (+) + Add
- red ✕ on tiles removes the tile; green + on empty space opens add-tile submenu
- 'Themes ▸' button opens a submenu with all ShaderPresets
- add-tile submenu populates from PluginManager.get_all_tile_defs()
- all toolbar buttons are 48×48 flat buttons with 24px font for touch
- orphan PopupPanel children cleaned up during grid rebuild
- plugin_manager: scans res://plugins/*/plugin.cfg, loads tile definitions
- plugin_tile: new base class for plugin tiles, extends ModuleBase
- layout_manager: save/restore tile grid positions per named layout
- migrate existing cpu/memory/testing tiles into system_monitor plugin
- add module_resized signal to DashboardGrid for auto-save
- dashboard reads from PluginManager + LayoutManager instead of hardcoded paths
- project.godot registers PluginManager and LayoutManager autoloads
- AGENTS.md updated with new structure and conventions
