Fix incomplete functionality, nullable warnings, and async issues

Incomplete functionality fixes:
- SkiaEditor: Wire up Completed event to fire on focus lost
- X11InputMethodService: Remove unused _commitCallback field
- SkiaWebView: Set _isProperlyReparented when reparenting succeeds,
  use _lastMainX/_lastMainY to track main window position,
  add _isEmbedded guard to prevent double embedding

Nullable reference fixes:
- Easing: Reorder BounceOut before BounceIn (static init order)
- GestureManager: Use local command variable instead of re-accessing
- SkiaShell: Handle null Title with ?? operator
- GLibNative: Use null! for closure pattern
- LinuxProgramHost: Default title if null
- SkiaWebView.LoadHtml: Add null/empty check for html
- SystemThemeService: Initialize Colors with default values
- DeviceDisplayService/AppInfoService: Use var for nullable env vars
- EmailService: Add null check for message parameter

Async fixes:
- SkiaImage: Use _ = for fire-and-forget async calls
- SystemTrayService: Convert async method without await to sync Task

Reduces warnings from 156 to 133 (remaining are P/Invoke structs
and obsolete MAUI API usage)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Services/SystemThemeService.cs

@@ -56,7 +56,19 @@ public class SystemThemeService
     /// <summary>
     /// System colors based on the current theme.
     /// </summary>
-    public SystemColors Colors { get; private set; }
+    public SystemColors Colors { get; private set; } = new SystemColors
+    {
+        Background = SKColors.White,
+        Surface = new SKColor(0xF5, 0xF5, 0xF5),
+        Primary = new SKColor(0x21, 0x96, 0xF3),
+        OnPrimary = SKColors.White,
+        Text = new SKColor(0x21, 0x21, 0x21),
+        TextSecondary = new SKColor(0x75, 0x75, 0x75),
+        Border = new SKColor(0xE0, 0xE0, 0xE0),
+        Divider = new SKColor(0xE0, 0xE0, 0xE0),
+        Error = new SKColor(0xF4, 0x43, 0x36),
+        Success = new SKColor(0x4C, 0xAF, 0x50)
+    };
 
     private FileSystemWatcher? _settingsWatcher;