fix: Correct cursor positioning and add window padding

Cursor Position Fix:
- Fix cursor screen position calculation to properly account for horizontal scrolling
- Ensure cursor aligns exactly with the character it's highlighting
- Cursor now correctly shows the character at cursor position, not offset characters

Window Padding Enhancement:
- Add 8px padding on all sides (left, top, right, bottom) for better visual layout
- Adjust viewport size calculations to account for padding area
- Update text rendering positions to start after left/top padding
- Update cursor position calculation to include padding offset
- Improve overall visual appearance and readability

Visual Improvements:
✅ Text no longer touches window edges
✅ Cursor perfectly aligned with characters
✅ Better visual spacing and professional appearance
✅ Consistent padding on window resize

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/gtk_editor.cpp

@@ -110,6 +110,12 @@ private:
     double line_height_ = 0;
     double ascent_ = 0;
     
+    // Layout padding
+    static constexpr double PADDING_LEFT = 8.0;
+    static constexpr double PADDING_TOP = 8.0;
+    static constexpr double PADDING_RIGHT = 8.0;
+    static constexpr double PADDING_BOTTOM = 8.0;
+    
     // Cursor blinking
     bool cursor_visible_ = true;
     sigc::connection cursor_timer_connection_;
@@ -177,9 +183,9 @@ protected:
         layout->get_pixel_extents(ink_rect, logical_rect);
         char_width_ = (double)logical_rect.get_width(); // Already in pixels, no PANGO_SCALE needed
 
-        // Update core's viewport size based on actual font metrics
-        int content_width_px = width; // Use actual width from parameter
-        int content_height_px = height; // Use actual height from parameter
+        // Update core's viewport size based on actual font metrics and padding
+        int content_width_px = width - static_cast<int>(PADDING_LEFT + PADDING_RIGHT);
+        int content_height_px = height - static_cast<int>(PADDING_TOP + PADDING_BOTTOM);
 
         int visible_lines = static_cast<int>(content_height_px / line_height_);
         int visible_cols = static_cast<int>(content_width_px / char_width_);
@@ -211,20 +217,21 @@ protected:
             layout->set_text(visible_text);
 
             // Render text at proper position (Cairo expects top-left, not baseline)
-            double text_y = screen_y * line_height_;
-            cr->move_to(0, text_y);
+            double text_x = PADDING_LEFT;
+            double text_y = PADDING_TOP + screen_y * line_height_;
+            cr->move_to(text_x, text_y);
             layout->show_in_cairo_context(cr);
         }
 
         // Render Cursor - Emacs-style blinking block cursor with color inversion
         if (cursor_visible_ && cursor.line >= static_cast<size_t>(start_line) && cursor.line < static_cast<size_t>(end_line)) {
-            // Account for horizontal scrolling
-            double cursor_screen_x = (static_cast<int>(cursor.column) - horizontal_offset) * char_width_;
+            // Account for horizontal scrolling and padding
+            double cursor_screen_x = PADDING_LEFT + (static_cast<int>(cursor.column) - horizontal_offset) * char_width_;
             int screen_y = cursor.line - start_line;
-            double cursor_y = screen_y * line_height_;
+            double cursor_y = PADDING_TOP + screen_y * line_height_;
             
             // Only render cursor if it's visible horizontally
-            if (cursor_screen_x >= 0 && cursor_screen_x < content_width_px) {
+            if (cursor_screen_x >= PADDING_LEFT && cursor_screen_x < (width - PADDING_RIGHT)) {
                 // Get the character under cursor for rendering with inverted colors
                 size_t buffer_line_idx = cursor.line;
                 const auto& cursor_line_text = buffer.line(buffer_line_idx);