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.

UndoSystem.h

@@ -12,7 +12,7 @@ class Buffer;
 
 class UndoSystem {
 public:
-    explicit UndoSystem(Buffer &owner, UndoTree &tree);
+	explicit UndoSystem(Buffer &owner, UndoTree &tree);
 
 	void Begin(UndoType type);
 
@@ -30,28 +30,30 @@ public:
 
 	void discard_pending();
 
- void clear();
+	void clear();
 
- void UpdateBufferReference(Buffer &new_buf);
+	void UpdateBufferReference(Buffer &new_buf);
 
 private:
-    void apply(const UndoNode *node, int direction); // +1 redo, -1 undo
-    void free_node(UndoNode *node);
+	void apply(const UndoNode *node, int direction); // +1 redo, -1 undo
+	void free_node(UndoNode *node);
 
-    void free_branch(UndoNode *node); // frees redo siblings only
-    UndoNode *find_parent(UndoNode *from, UndoNode *target);
+	void free_branch(UndoNode *node); // frees redo siblings only
+	UndoNode *find_parent(UndoNode *from, UndoNode *target);
 
-    // Debug helpers (compiled only when KTE_UNDO_DEBUG is defined)
-    void debug_log(const char *op) const;
-    static const char *type_str(UndoType t);
-    static bool is_descendant(UndoNode *root, const UndoNode *target);
+	// Debug helpers (compiled only when KTE_UNDO_DEBUG is defined)
+	void debug_log(const char *op) const;
 
-    void update_dirty_flag();
+	static const char *type_str(UndoType t);
+
+	static bool is_descendant(UndoNode *root, const UndoNode *target);
+
+	void update_dirty_flag();
 
 	Buffer *buf_;
 	UndoTree &tree_;
-    // Internal hint for Delete batching: whether next Append() should prepend
-    bool pending_prepend_ = false;
+	// Internal hint for Delete batching: whether next Append() should prepend
+	bool pending_prepend_ = false;
 };
 
 #endif // KTE_UNDOSYSTEM_H