fix: use python3 in delete-files makefile macro

macOS ships no `python` binary (Python 2 was removed in 12.3), and modern
Debian/Ubuntu no longer provide the unversioned name either, so the
delete-files macro failed with "command not found" and broke webui-dist.

Matches the existing python3 usage for scripts/devcheck.py.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012G2QtanzeybTQdJLZHtiJQ

Makefile

@@ -1,5 +1,5 @@
 define delete-files
-	python -c "import shutil;shutil.rmtree('$(1)', ignore_errors=True)"
+	python3 -c "import shutil;shutil.rmtree('$(1)', ignore_errors=True)"
 endef
 
 service:

frontend/Makefile

@@ -1,5 +1,5 @@
 define delete-files
-	python -c "import shutil;shutil.rmtree('$(1)', ignore_errors=True)"
+	python3 -c "import shutil;shutil.rmtree('$(1)', ignore_errors=True)"
 endef
 
 codestyle:

service/Makefile

@@ -1,5 +1,5 @@
 define delete-files
-	python -c "import shutil;shutil.rmtree('$(1)', ignore_errors=True)"
+	python3 -c "import shutil;shutil.rmtree('$(1)', ignore_errors=True)"
 endef
 
 compile-currentenv: