feat: add 13 color configuration fields for panel, workspace, title, border, and notification themes

Add color customization support to the Config struct with 13 new char[16] fields covering panel background/foreground, workspace active/inactive/urgent, title focused/unfocused background/foreground, border focused/unfocused, and notification background/foreground. Implement default value initialization in config_set_defaults using strncpy with DEFAULT_* constants, and add parsing logic in handle_config_entry for the new "colors" section to populate these fields from configuration files.

include/config.h

@@ -67,6 +67,20 @@ struct Config {
     int demo_step_delay_ms;
     int demo_ai_timeout_ms;
     int demo_window_timeout_ms;
+
+    char color_panel_bg[16];
+    char color_panel_fg[16];
+    char color_workspace_active[16];
+    char color_workspace_inactive[16];
+    char color_workspace_urgent[16];
+    char color_title_focused_bg[16];
+    char color_title_focused_fg[16];
+    char color_title_unfocused_bg[16];
+    char color_title_unfocused_fg[16];
+    char color_border_focused[16];
+    char color_border_unfocused[16];
+    char color_notification_bg[16];
+    char color_notification_fg[16];
 };
 
 Config *config_create(void);