Prevent \uXXXX escape leakage on paste and enforce UTF-8 song file persistence

[Copilot]

Pasting into the song editor could persist non-ASCII text as escaped Unicode literals (e.g. \u4F60\u597D) instead of real characters. This change ensures pasted content is materialized as Unicode text and song files are consistently read/written as UTF-8.

• Editor paste decoding

  • Added decoding for \uXXXX sequences during paste handling in TfrmDisplaySongContent.
  • Handles valid surrogate pairs (\uD800..\uDBFF + \uDC00..\uDFFF) and leaves invalid surrogate input unchanged.

• Explicit UTF-8 file I/O in editor flows

  • Switched editor load/save paths to explicit TEncoding.UTF8 in editordisplaysongcontent.pas.
  • Applied UTF-8 writes to song copy/new-save related flow in songeditor.pas.

• UTF-8 consistency in song/text persistence helpers

  • Updated song export path in lyrics.pas to write UTF-8 explicitly.
  • Updated generic text file wrapper in textfilehandler.pas to load/save with UTF-8.

procedure TfrmDisplaySongContent.memoCodePaste(Sender: TObject;
  var AText: String; var AMode: TSynSelectionMode; ALogStartPos: TPoint;
  var AnAction: TSynCopyPasteAction);
begin
  AText := DecodeEscapedUnicodeSequences(AText);
end;

memoCode.Lines.LoadFromFile(self.openFilePath, TEncoding.UTF8);
memoCode.Lines.SaveToFile(self.openFilePath, TEncoding.UTF8);

[Copilot]

Pull request overview

This PR aims to prevent pasted \uXXXX escape sequences from being persisted literally in the song editor, and to make song/text persistence consistently use explicit UTF-8 encoding across editor and helper flows.

Changes:

• Added a paste-time decoder that converts \uXXXX (including surrogate pairs) into actual Unicode characters.
• Switched editor load/save (and copy flow) to explicit TEncoding.UTF8.
• Updated song export and generic text file helpers to read/write with TEncoding.UTF8.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/generics/textfilehandler.pas	Uses explicit UTF-8 for generic text load/save.
src/generics/lyrics.pas	Exports song files explicitly as UTF-8.
src/frames/editordisplaysongcontent.pas	Adds \uXXXX paste decoding; editor load/save now explicit UTF-8.
src/forms/songeditor.pas	Copy-to-new-song write now explicit UTF-8.

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