feat(client): add support for 8-bit hex color codes in CSS

This allows users to use #RRGGBBAA format in their style.css.
The client now detects 8-bit hex codes, transforms them into
GTK-compatible rgba() syntax, and loads the modified data
into the CSS provider.

- Added utility to detect 8-bit hex patterns.
- Added transformation logic to convert hex alpha to decimal.
- Intercepted CSS loading in Client::setupCss to handle the conversion.

include/util/hex_checker.hpp

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <string>
+
+/**
+ * Reads a CSS file, searches for 8-bit hex codes (#RRGGBBAA),
+ * and returns true if found, or false if not found.
+ */
+
+bool has_8bit_hex(std::string file_path);
+
+/**
+ * Reads a CSS file, searches for 8-bit hex codes (#RRGGBBAA),
+ * and transforms them into GTK-compatible rgba() syntax.
+ */
+
+std::string transform_8bit_to_hex(std::string file_path);