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/tiles/cpu/cpu_tile.tscn

@@ -0,0 +1,41 @@
+[gd_scene format=3 uid="uid://bq3bs2hb4r7fb"]
+
+[ext_resource type="Script" path="res://plugins/local_system_monitor/tiles/cpu/cpu_tile.gd" id="1"]
+[ext_resource type="Shader" path="res://shaders/vial_fill.gdshader" id="2"]
+
+[node name="CpuTile" type="PanelContainer"]
+anchors_preset = 0
+script = ExtResource("1")
+
+[node name="VialFill" type="ColorRect" parent="."]
+unique_name_in_owner = true
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+mouse_filter = 2
+
+[node name="MarginContainer" type="MarginContainer" parent="."]
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+theme_constant_overrides/margin_left = 12
+theme_constant_overrides/margin_top = 12
+theme_constant_overrides/margin_right = 12
+theme_constant_overrides/margin_bottom = 12
+
+[node name="VBoxContainer" type="VBoxContainer" parent="MarginContainer"]
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+
+[node name="Title" type="Label" parent="MarginContainer/VBoxContainer"]
+unique_name_in_owner = true
+text = "CPU"
+
+[node name="Spacer" type="Control" parent="MarginContainer/VBoxContainer"]
+size_flags_vertical = 3
+
+[node name="Label" type="Label" parent="MarginContainer/VBoxContainer"]
+unique_name_in_owner = true
+horizontal_alignment = 1
+size_flags_horizontal = 4