Fix font atlas crash and make Nix build first-class

Defer edit-mode font switch to next frame via RequestLoadFont() to
avoid modifying the locked ImFontAtlas between NewFrame() and Render().

Rework Nix packaging: split nativeBuildInputs/buildInputs correctly,
add devShells for nix develop, desktop file for kge, per-variant pname
and meta.mainProgram, and an overlay for NixOS configs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ImGuiFrontend.cc

@@ -565,7 +565,7 @@ GUIFrontend::Step(Editor &ed, bool &running)
 			running = false;
 		}
 
-		// Switch font based on current buffer's edit mode
+		// Switch font based on current buffer's edit mode (deferred to next frame)
 		{
 			Buffer *cur = wed.CurrentBuffer();
 			if (cur) {