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.

meson.build

@@ -185,7 +185,9 @@ src_files = files(
     'src/util/gtk_icon.cpp',
     'src/util/icon_loader.cpp',
     'src/util/regex_collection.cpp',
-    'src/util/css_reload_helper.cpp'
+    'src/util/css_reload_helper.cpp',
+    'src/util/has_8bit_hex.cpp',
+    'src/util/transform_8bit_to_rgba.cpp'
 )
 
 man_files = files(