add plugin settings, per-tile refresh, settings menu, touch-friendly UI

- Plugin settings infrastructure in PluginManager with config/plugin_settings.cfg
  storage, manifest [settings] sections, CRUD API, plugin_setting_changed signal
- Per-tile refresh intervals: 100ms base tick, per-tile update_interval_ms
  metadata, dynamic tween durations clamped to refresh window
- Settings menu (PopupPanel) with General/Plugins/About tabs, plugin
  activation toggles, per-plugin settings cog buttons
- Plugin settings popup (PopupPanel) with dynamic UI from setting definitions
  (SpinBox/CheckBox/LineEdit), auto-save on change
- Modal behavior: exclusive = true on settings windows
- Touch-friendly sizing: enlarged close buttons, cog buttons, controls, spacing
- Red corner close button redesign with rounded corners, hover/pressed states
- Split system_monitor into local_system_monitor (CPU, Memory) and test plugins
- Plugin activation/deactivation with layout preservation

plugins/local_system_monitor/plugin.cfg

@@ -0,0 +1,49 @@
+[plugin]
+name="Local System Monitor"
+version="0.1.0"
+description="Monitors local system resources: CPU and memory"
+author="Fifthdread"
+
+[tiles]
+count=2
+
+[tile_0]
+id="cpu"
+name="CPU Monitor"
+scene="res://plugins/local_system_monitor/tiles/cpu/cpu_tile.tscn"
+min_w=1
+min_h=1
+max_w=4
+max_h=4
+default_w=1
+default_h=1
+
+[tile_1]
+id="memory"
+name="Memory Monitor"
+scene="res://plugins/local_system_monitor/tiles/memory/memory_tile.tscn"
+min_w=1
+min_h=1
+max_w=4
+max_h=4
+default_w=1
+default_h=1
+
+[settings]
+count=2
+
+[setting_0]
+key="cpu_update_interval_ms"
+label="CPU Update Interval"
+type="int"
+default=1000
+min=100
+max=60000
+
+[setting_1]
+key="memory_update_interval_ms"
+label="Memory Update Interval"
+type="int"
+default=1000
+min=100
+max=60000