rework dev server: hmr for all courses, per-course targets, root index

- make dev serves all courses on port 1312 with marp live reload
- make dev-b/dev-c for single course on 1313/1314
- dev-kill only kills specific ports instead of all marp processes
- add generate-root-index.sh for prod build overview page
- cleanup function uses exec + trap instead of pid files

Makefile

@@ -27,7 +27,9 @@ help:
 	@echo "  223015c - Internettechnologien"
 	@echo ""
 	@echo "Development:"
-	@echo "  make dev         - Start development server (port 3000)"
+	@echo "  make dev         - Marp live server all courses (port 1312, HMR)"
+	@echo "  make dev-b       - Marp live server 223015b (port 1313, HMR)"
+	@echo "  make dev-c       - Marp live server 223015c (port 1314, HMR)"
 	@echo ""
 	@echo "Build:"
 	@echo "  make build       - Build all courses"
@@ -55,13 +57,20 @@ build/.exists:
 	@mkdir -p build/223015b build/223015c
 	@touch $@
 
-# Development server
+# Development server (Marp live server with HMR)
 dev:
 	@./scripts/dev-server.sh
 
+dev-b:
+	@./scripts/dev-server.sh 223015b 1313
+
+dev-c:
+	@./scripts/dev-server.sh 223015c 1314
+
 dev-kill:
-	@-pkill -f "marp-cli.*--server" 2>/dev/null || true
-	@sleep 0.5
+	@-fuser -k 1312/tcp 2>/dev/null || true
+	@-fuser -k 1313/tcp 2>/dev/null || true
+	@-fuser -k 1314/tcp 2>/dev/null || true
 
 # Build functions
 define build_course