Save imgui.ini to %APPDATA%\UsdLayerManager\ instead of CWD

Sets io.IniFilename to %APPDATA%\UsdLayerManager\imgui.ini right after
CreateContext so ImGui never touches the working directory for layout
persistence. The directory is created on first run via create_directories.
m_iniFilePath on ImGuiContext holds the string so the pointer stays valid
for the lifetime of the context.

Also removes imgui.ini from version control and adds it to .gitignore.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui/ImGuiContext.cpp

@@ -5,6 +5,7 @@
 #include <imgui.h>
 #include <imgui_impl_win32.h>
 #include <imgui_impl_opengl3.h>
+#include <filesystem>
 
 // Forward declare message handler from imgui_impl_win32.cpp
 extern IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);
@@ -94,6 +95,15 @@ bool ImGuiContext::Initialize(const std::string& windowTitle, int width, int hei
     // ImGui window is focused, which would block all viewport hotkeys (F, A, etc.).
     io.ConfigFlags |= ImGuiConfigFlags_DockingEnable;
 
+    // Store imgui.ini in %APPDATA%\UsdLayerManager\ so it doesn't litter the CWD.
+    if (const char* appData = getenv("APPDATA")) {
+        std::filesystem::path iniDir =
+            std::filesystem::path(appData) / "UsdLayerManager";
+        std::filesystem::create_directories(iniDir);
+        m_iniFilePath = (iniDir / "imgui.ini").string();
+        io.IniFilename = m_iniFilePath.c_str();
+    }
+
     // Load Inter font.
     // The build directory copies the font as Inter.ttc; the install step renames
     // it to Inter.ttf.  Try .ttf first (install layout), fall back to .ttc (build).