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

config/layouts/Main.cfg

@@ -1,7 +1,9 @@
 [layout]
 
 name="Main"
-saved_at="2026-05-21T09:16:49"
+saved_at="2026-05-21T12:08:02"
+grid_columns=4
+grid_rows=3
 
 [tiles]
 
@@ -9,24 +11,24 @@ count=3
 
 [tile_0]
 
-id="system_monitor/cpu"
-col=1
+id="local_system_monitor/cpu"
+col=3
 row=0
 w=1
 h=1
 
 [tile_1]
 
-id="system_monitor/memory"
-col=2
+id="local_system_monitor/memory"
+col=1
 row=0
 w=1
 h=1
 
 [tile_2]
 
-id="system_monitor/testing"
-col=0
+id="test/testing"
+col=2
 row=0
 w=1
-h=3
+h=1