feat(editor): implement dedicated HierarchyPanel with search, context menu, delete key

[LyeZinho]

Summary

Implement a dedicated HierarchyPanel class (issue #76, Milestone M1) with its own .cpp file, refactored from the previous inline header.

Changes

New file:

• src/editor/HierarchyPanel.cpp — Full ImGui rendering implementation

Modified files:

• src/editor/HierarchyPanel.hpp — Refactored from 189-line inline-heavy header to clean class declaration; now stores EditorContext* and World* as members
• src/editor/SceneEditor.hpp — SceneEditor() constructor passes &m_ctx to HierarchyPanel
• CMakeLists.txt — Added HierarchyPanel.cpp to doppio executable sources
• tests/test_editor.cpp — Added 4 tests for panel construction/state

Features per acceptance criteria

• ✅ Root entities in tree; children indented under parent
• ✅ Click selects entity in EditorContext
• ✅ Right-click empty space → Create Empty Entity
• ✅ Delete key with focus removes selected entity
• ✅ Search filter (case-insensitive substring match)
• ✅ Entity context menu: Rename, Create Child, Delete
• ✅ Drag-and-drop reparenting
• ✅ All operations wrapped in beginUndo/endUndo