Add 5 new tools: QR-Code, Markdown, Color Converter, JSON Formatter, Regex Tester

- backend/tools/qrcode_gen.py: POST /api/qrcode/generate, returns base64 PNG (qrcode[pil])
- backend/tools/markdown_tool.py: POST /api/markdown/render, extensions: tables/fenced_code/nl2br
- backend/tools/colorconverter.py: POST /api/color/convert, HEX/RGB/HSL via colorsys (no deps)
- backend/tools/jsonformatter.py: POST /api/json/format, returns formatted JSON with line/col errors
- backend/tools/regextester.py: POST /api/regex/test, flags i/m/s, returns matches with positions
- QrCodeTool.jsx: generate + download PNG button
- MarkdownTool.jsx: split editor/preview, debounce 500ms, white preview bg
- ColorConverterTool.jsx: color swatch preview, per-format copy buttons
- JsonFormatterTool.jsx: indent toggle 2/4, pre result box with copy
- RegexTesterTool.jsx: debounce 400ms, yellow match highlighting, flag checkboxes
- All blueprints registered in app.py; qrcode[pil] + markdown added to requirements.txt

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

backend/tools/markdown_tool.py

@@ -0,0 +1,21 @@
+from flask import Blueprint, request, jsonify
+import markdown
+from util.logger import logger
+from auth.token import verify_token
+
+markdown_blueprint = Blueprint('markdown_tool', __name__)
+
+
+@markdown_blueprint.route('/api/markdown/render', methods=['POST'])
+def render_markdown():
+    user = verify_token()
+    if not user:
+        return jsonify({"message": "Nicht autorisiert"}), 401
+    try:
+        data = request.get_json(silent=True) or {}
+        text = data.get("text", "")
+        html = markdown.markdown(text, extensions=["tables", "fenced_code", "nl2br"])
+        return jsonify({"html": html})
+    except Exception as e:
+        logger.error(f"Fehler Markdown: {e}")
+        return jsonify({"message": "Fehler beim Rendern"}), 500