Upgrade to .NET 10 and resolve all nullable warnings

[Aswikinz]

Upgrade target framework from net6.0-windows to net10.0-windows and add EnableWindowsTargeting for cross-platform build support.Fixed 20 compiler warnings (0 errors) to achieve a clean build.

Target framework:

• SymbolPicker.csproj: net6.0-windows → net10.0-windows
• Add EnableWindowsTargeting property for building on non-Windows hosts

Nullable reference type fixes (CS8618, CS8600, CS8604, CS8625, CS8602):

• Program.cs: Initialize SettingPage and MainForm with null! suppression since they are assigned in Main() before any access
• Program.cs: Annotate FindWindow lpClassName parameter as string?
• Form1.cs: Make CancellationTokenSource field nullable
• Form1.cs: Replace unsafe (Button)sender cast with pattern match (if sender is not Button senderButton) for null safety
• Form1.cs: Use Symbol? and Button? for FirstOrDefault() return values
• Form1.cs: Guard addS?.img dereference with fallback to senderButton.Text
• Form1.cs: Pass actual sender/e args instead of null literals to textBox_search_Leave from keyboard handlers
• Form1.cs: Use MouseEventArgs? for the as-cast in tray click handler
• Settings.cs: Use Dictionary<string, object>? for Deserialize result
• Settings.cs: Use object? for Convert.ChangeType and GetValue results

Naming (CS8981):

• Form1.cs: Rename HotKey.control enum to HotKey.HotkeyModifier to avoid lowercase-only type name that may conflict with future C# keywords

Unused code (CS0169):

• Settings.Designer.cs: Remove unused checkBox2 field