Refine help text, keybindings, GUI themes, and undo system.

- Expanded help text and command documentation with detailed keybinding descriptions.
- Added theme customization support to GUIConfig (Nord default, light/dark variants).
- Adjusted for consistent indentation and debug instrumentation in undo system.
- Enhanced test cases for multi-line, UTF-8, and branching scenarios.

Buffer.h

@@ -16,7 +16,7 @@
 
 class Buffer {
 public:
-    Buffer();
+	Buffer();
 
 	Buffer(const Buffer &other);
 
@@ -262,11 +262,12 @@ public:
 		return filename_;
 	}
 
+
 	// Set a virtual (non file-backed) display name for this buffer, e.g. "+HELP+"
 	// This does not mark the buffer as file-backed.
 	void SetVirtualName(const std::string &name)
 	{
-		filename_ = name;
+		filename_       = name;
 		is_file_backed_ = false;
 	}
 
@@ -277,26 +278,29 @@ public:
 	}
 
 
- [[nodiscard]] bool Dirty() const
- {
-     return dirty_;
- }
+	[[nodiscard]] bool Dirty() const
+	{
+		return dirty_;
+	}
 
- // Read-only flag
- [[nodiscard]] bool IsReadOnly() const
- {
-     return read_only_;
- }
 
- void SetReadOnly(bool ro)
- {
-     read_only_ = ro;
- }
+	// Read-only flag
+	[[nodiscard]] bool IsReadOnly() const
+	{
+		return read_only_;
+	}
 
- void ToggleReadOnly()
- {
-     read_only_ = !read_only_;
- }
+
+	void SetReadOnly(bool ro)
+	{
+		read_only_ = ro;
+	}
+
+
+	void ToggleReadOnly()
+	{
+		read_only_ = !read_only_;
+	}
 
 
 	void SetCursor(const std::size_t x, const std::size_t y)
@@ -380,18 +384,18 @@ public:
 	[[nodiscard]] const UndoSystem *Undo() const;
 
 private:
-    // State mirroring original C struct (without undo_tree)
-    std::size_t curx_    = 0, cury_ = 0; // cursor position in characters
-    std::size_t rx_      = 0; // render x (tabs expanded)
-    std::size_t nrows_   = 0; // number of rows
+	// State mirroring original C struct (without undo_tree)
+	std::size_t curx_    = 0, cury_ = 0; // cursor position in characters
+	std::size_t rx_      = 0; // render x (tabs expanded)
+	std::size_t nrows_   = 0; // number of rows
 	std::size_t rowoffs_ = 0, coloffs_ = 0; // viewport offsets
 	std::vector<Line> rows_; // buffer rows (without trailing newlines)
 	std::string filename_;
- bool is_file_backed_   = false;
- bool dirty_            = false;
- bool read_only_        = false;
- bool mark_set_         = false;
- std::size_t mark_curx_ = 0, mark_cury_ = 0;
+	bool is_file_backed_   = false;
+	bool dirty_            = false;
+	bool read_only_        = false;
+	bool mark_set_         = false;
+	std::size_t mark_curx_ = 0, mark_cury_ = 0;
 
 	// Per-buffer undo state
 	std::unique_ptr<struct UndoTree> undo_tree_;