feat(ui): implement click-to-move cursor

Enhanced mouse click handler to calculate line/column from coordinates and move the cursor to the clicked position, including boundary checks and window activation.

src/gtk_editor.cpp

@@ -420,18 +420,42 @@ protected:
             }, false);
             drawing_area->add_controller(controller);
             
-            // Add click handling to set active window explicitly
+            // Add click handling to set active window explicitly and move cursor
             // We use GestureClick instead of EventControllerFocus to avoid spurious focus changes
-            // (e.g. caused by layout rebuilds or mouse hover anomalies)
             auto click_controller = Gtk::GestureClick::create();
             std::weak_ptr<Window> weak_window_click = node->window;
-            click_controller->signal_pressed().connect([this, weak_window_click, drawing_area](int, double, double) {
+            click_controller->signal_pressed().connect([this, weak_window_click, drawing_area](int /*n_press*/, double x, double y) {
                 if (auto window = weak_window_click.lock()) {
+                    // 1. Activate Window
                     if (core_ && core_->active_window() != window) {
                         core_->set_active_window(window);
                     }
                     // IMPORTANT: Grab keyboard focus for this widget
                     drawing_area->grab_focus();
+                    
+                    // 2. Move Cursor
+                    if (line_height_ > 0 && char_width_ > 0) {
+                        int row = static_cast<int>((y - PADDING_TOP) / line_height_);
+                        int col = static_cast<int>((x - PADDING_LEFT) / char_width_);
+                        
+                        if (row < 0) row = 0;
+                        if (col < 0) col = 0;
+                        
+                        auto viewport = window->viewport();
+                        size_t target_line = viewport.scroll_offset + row;
+                        size_t target_col = viewport.horizontal_offset + col;
+                        
+                        // Check boundaries
+                        if (target_line < window->buffer().line_count()) {
+                             window->set_cursor({target_line, target_col});
+                             drawing_area->queue_draw();
+                        } else {
+                             // Clicked past end of buffer, move to end
+                             size_t last_line = window->buffer().line_count() - 1;
+                             window->set_cursor({last_line, window->buffer().line(last_line).length()});
+                             drawing_area->queue_draw();
+                        }
+                    }
                 }
             });
             drawing_area->add_controller(click_controller);