ui : embed assets directly with CMake (#28445)

Remove the build-time C++ helper and external gzip dependency,
simplifying cross-compilation. Keep the generated C++ in templates for
readability and preserve fully embedded UI assets.

Signed-off-by: Adrien Gallouët <angt@huggingface.co>

tools/ui/ui.h.in

@@ -0,0 +1,21 @@
+// Generated by scripts/ui-assets.cmake - do not edit.
+
+#pragma once
+
+#include <array>
+#include <string>
+
+// Defined as 1 only when assets were embedded (tools/server checks defined()).
+#cmakedefine LLAMA_UI_HAS_ASSETS 1
+
+struct llama_ui_asset {
+    std::string           name;
+    const unsigned char * data;
+    std::size_t           size;
+    std::string           etag;
+    std::string           type;
+};
+
+const llama_ui_asset * llama_ui_find_asset(const std::string & name);
+bool llama_ui_use_gzip();
+const std::array<llama_ui_asset, @N_ASSETS@> & llama_ui_get_assets();