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/app.py

@@ -18,6 +18,11 @@ from tools import (
     passwordgen_blueprint,
     timestamp_blueprint,
     textdiff_blueprint,
+    qrcode_blueprint,
+    markdown_blueprint,
+    color_blueprint,
+    json_formatter_blueprint,
+    regex_blueprint,
 )
 from admin import admin_bp
 
@@ -34,6 +39,11 @@ app.register_blueprint(jwt_decoder_blueprint)
 app.register_blueprint(passwordgen_blueprint)
 app.register_blueprint(timestamp_blueprint)
 app.register_blueprint(textdiff_blueprint)
+app.register_blueprint(qrcode_blueprint)
+app.register_blueprint(markdown_blueprint)
+app.register_blueprint(color_blueprint)
+app.register_blueprint(json_formatter_blueprint)
+app.register_blueprint(regex_blueprint)
 app.register_blueprint(admin_bp)
 
 # 🌐 React-Frontend ausliefern