refactor(ui): replace Text with Label/Widget component system

Replace the old `Text` class with a new component-based UI system consisting of `Widget` and `Label` classes. Key changes:

- Remove `Text` class and its GPU upload logic; introduce `Label` as a `Widget` subclass with dirty-flag vertex update.
- Add `UIVertexData` struct to manage per-label vertex buffer/array and memory.
- Refactor `DebugCollector` to store labels in a `Widget` tree, removing hash-based lookup.
- Simplify `Font::vertices()` signature by removing position/scale parameters; scaling is now applied in the renderer’s model matrix.
- Update `Renderer` with `render_label()` and move UI rendering to a `render_ui()` that renders the widget tree.
- Add new source files `label.cpp`, `widget.cpp`, `ui_vertex_data.cpp` and update CMakeLists.

src/CMakeLists.txt

@@ -17,7 +17,6 @@ target_sources(${PROJECT_NAME}
         tools/shader_tools.cpp
         tools/font.cpp
         tools/perlin_noise.cpp
-        ui/text.cpp
         window.cpp
         gameplay/builders/biome_builder.cpp
         gameplay/builders/plain_builder.cpp
@@ -59,4 +58,7 @@ target_sources(${PROJECT_NAME}
         render/frame_buffer.cpp
         render/shader_manager.cpp
         render/world_renderer.cpp
+        ui/widget.cpp
+        ui/label.cpp
+        ui/ui_vertex_data.cpp
 )