Standardized GTK3 menu-related icon constants to the bundled zc-menu-* namespace so menu actions stop depending on mixed system-theme icon names (edit-copy, view-refresh, etc.) and instead use the same internal set. This was updated in channel list, plugin manager, URL grabber, and spell-entry menu definitions.

Updated GTK3 icon rendering paths in affected UI helpers to call gtkutil_image_new_from_stock(...), which is the path that resolves zc-menu-* resources correctly and consistently from data/icons/menu (instead of raw theme lookup).

Added new icons to the shared bundled menu icon set (light/dark) for copy, delete, add, remove, spell-check, and refresh, and registered them in the GResource manifest so they are always available at runtime.

data/icons/menu/dark/delete.svg

@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16" aria-hidden="true">
+<path d="M5.2 5.3h5.6v7.2H5.2z" fill="none" stroke="#e5e7eb" stroke-width="1.3"/><path d="M4 5.3h8" stroke="#e5e7eb" stroke-width="1.3" stroke-linecap="round"/><path d="M6.3 3.7h3.4" stroke="#e5e7eb" stroke-width="1.3" stroke-linecap="round"/><path d="M7.1 6.6v4.6M8.9 6.6v4.6" stroke="#e5e7eb" stroke-width="1.1" stroke-linecap="round"/>
+</svg>