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.gd

@@ -0,0 +1,71 @@
+extends PluginTile
+
+
+@onready var title_label: Label = %Title
+@onready var label: Label = %Label
+@onready var vial_fill: ColorRect = %VialFill
+
+var _collector: CpuCollector
+var _displayed_fill: float = 0.0
+var _fill_tween: Tween
+
+
+func initialize() -> void:
+	module_title = "CPU"
+	_collector = CpuCollector.new()
+	_setup_shader()
+	_style()
+
+
+func refresh(data: Dictionary) -> void:
+	var usage: float = _collector.collect()
+	if usage < 0.0:
+		return
+
+	var pct: int = roundi(usage)
+	label.text = "%d%%" % pct
+
+	# Smoothly tween the vial fill — clamp duration so animation never outlasts the refresh interval
+	var target: float = usage / 100.0
+	var update_ms: int = data.get("update_interval_ms", 1000)
+	var duration: float = minf(0.4, update_ms / 2000.0)
+	if _fill_tween and _fill_tween.is_valid():
+		_fill_tween.kill()
+	_fill_tween = create_tween()
+	_fill_tween.tween_method(_set_fill, _displayed_fill, target, duration).set_ease(Tween.EASE_OUT).set_trans(Tween.TRANS_CUBIC)
+
+
+func _set_fill(value: float) -> void:
+	_displayed_fill = value
+	var mat := vial_fill.material as ShaderMaterial
+	if mat:
+		mat.set_shader_parameter("fill", value)
+
+
+func _setup_shader() -> void:
+	var mat := ShaderMaterial.new()
+	mat.shader = preload("res://shaders/vial_fill.gdshader")
+	mat.set_shader_parameter("liquid_color", Color(0.2, 0.5, 0.8, 1.0))
+	mat.set_shader_parameter("fill", 0.0)
+	mat.set_shader_parameter("noise_tex", load("res://assets/textures/noise_100.png"))
+	vial_fill.material = mat
+
+
+func _style() -> void:
+	# Transparent root panel — the VialFill shader provides the background
+	var panel := StyleBoxFlat.new()
+	panel.bg_color = Color.TRANSPARENT
+	panel.corner_radius_top_left = 12
+	panel.corner_radius_top_right = 12
+	panel.corner_radius_bottom_right = 12
+	panel.corner_radius_bottom_left = 12
+	add_theme_stylebox_override("panel", panel)
+	add_theme_stylebox_override("panel_focused", panel)
+
+	title_label.add_theme_color_override("font_color", Color(0.7, 0.7, 0.8, 1.0))
+	title_label.add_theme_constant_override("outline_size", 2)
+	title_label.add_theme_color_override("font_outline_color", Color(0.0, 0.0, 0.0, 0.5))
+	label.add_theme_color_override("font_color", Color(0.9, 0.9, 1.0, 1.0))
+	label.add_theme_font_size_override("font_size", 48)
+	label.add_theme_constant_override("outline_size", 3)
+	label.add_theme_color_override("font_outline_color", Color(0.0, 0.0, 0.0, 0.7))