add nix flake with dev tools

provides:
- nix develop: dev shell with nodejs, qrencode, imagemagick
- nix run .#qr: generate qr codes from urls
- nix run .#qr-slides: generate qr for course urls
- nix run .#optimize-img: resize images to max width

flake.nix

@@ -0,0 +1,122 @@
+{
+  description = "HdM Slides - Marp presentation builder with tools";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+
+        # QR code generator script
+        qr-gen = pkgs.writeShellScriptBin "qr" ''
+          if [ -z "$1" ]; then
+            echo "Usage: qr <url> [output.png]"
+            echo ""
+            echo "Examples:"
+            echo "  qr https://example.com"
+            echo "  qr https://example.com my-qr.png"
+            echo "  qr 'https://librete.ch/hdm/223015b/' slides-qr.png"
+            exit 1
+          fi
+
+          URL="$1"
+          OUTPUT="''${2:-qr-code.png}"
+
+          ${pkgs.qrencode}/bin/qrencode -o "$OUTPUT" -s 10 -m 2 "$URL"
+          echo "Generated: $OUTPUT"
+        '';
+
+        # QR code for course slides
+        qr-slides = pkgs.writeShellScriptBin "qr-slides" ''
+          COURSE="''${1:-223015b}"
+          OUTPUT="''${2:-build/$COURSE/qr-$COURSE.png}"
+
+          mkdir -p "$(dirname "$OUTPUT")"
+          ${pkgs.qrencode}/bin/qrencode -o "$OUTPUT" -s 10 -m 2 "https://librete.ch/hdm/$COURSE/"
+          echo "Generated QR for $COURSE: $OUTPUT"
+        '';
+
+        # Image optimizer script
+        optimize-img = pkgs.writeShellScriptBin "optimize-img" ''
+          if [ -z "$1" ]; then
+            echo "Usage: optimize-img <image-or-directory> [max-width]"
+            echo ""
+            echo "Resizes images to max width (default: 1920px)"
+            exit 1
+          fi
+
+          TARGET="$1"
+          MAX_WIDTH="''${2:-1920}"
+
+          if [ -d "$TARGET" ]; then
+            for img in "$TARGET"/*.{png,jpg,jpeg,webp} 2>/dev/null; do
+              [ -f "$img" ] || continue
+              echo "Optimizing: $(basename "$img")"
+              ${pkgs.imagemagick}/bin/magick "$img" -resize "''${MAX_WIDTH}x>" -quality 85 "$img"
+            done
+          elif [ -f "$TARGET" ]; then
+            echo "Optimizing: $TARGET"
+            ${pkgs.imagemagick}/bin/magick "$TARGET" -resize "''${MAX_WIDTH}x>" -quality 85 "$TARGET"
+          else
+            echo "Error: $TARGET not found"
+            exit 1
+          fi
+          echo "Done!"
+        '';
+
+      in {
+        # Development shell with all tools
+        devShells.default = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            nodejs_20
+            nodePackages.npm
+            qrencode
+            imagemagick
+            gnumake
+          ];
+
+          shellHook = ''
+            echo "HdM Slides Development Environment"
+            echo ""
+            echo "Available commands:"
+            echo "  make dev-b     - Start 223015b dev server (port 1312)"
+            echo "  make dev-c     - Start 223015c dev server (port 1313)"
+            echo "  make build     - Build all courses"
+            echo "  qr <url>       - Generate QR code"
+            echo "  qr-slides 223015b - Generate QR for course"
+            echo "  optimize-img <dir> - Optimize images"
+            echo ""
+          '';
+        };
+
+        # Standalone apps
+        packages = {
+          qr = qr-gen;
+          qr-slides = qr-slides;
+          optimize-img = optimize-img;
+          default = qr-gen;
+        };
+
+        # Direct runnable apps
+        apps = {
+          qr = {
+            type = "app";
+            program = "${qr-gen}/bin/qr";
+          };
+          qr-slides = {
+            type = "app";
+            program = "${qr-slides}/bin/qr-slides";
+          };
+          optimize-img = {
+            type = "app";
+            program = "${optimize-img}/bin/optimize-img";
+          };
+          default = self.apps.${system}.qr;
+        };
+      }
+    );
+}