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>

frontend/src/components/ToolOverview.jsx

@@ -10,6 +10,11 @@ const TOOLS = [
   { icon: '🔐', path: '/tools/password',  title: 'Passwort-Generator',   desc: 'Sichere Passwörter generieren' },
   { icon: '🕐', path: '/tools/timestamp', title: 'Timestamp Converter',  desc: 'Unix Timestamp in Datum umrechnen' },
   { icon: '📝', path: '/tools/textdiff',  title: 'Text Diff',            desc: 'Zwei Texte vergleichen' },
+  { icon: '📷', path: '/tools/qrcode',   title: 'QR-Code Generator',    desc: 'Text oder URL als QR-Code generieren' },
+  { icon: '📄', path: '/tools/markdown', title: 'Markdown Editor',      desc: 'Markdown live rendern und vorschauen' },
+  { icon: '🎨', path: '/tools/color',    title: 'Farb-Konverter',       desc: 'HEX, RGB und HSL konvertieren' },
+  { icon: '{ }', path: '/tools/json',   title: 'JSON Formatter',       desc: 'JSON formatieren und validieren' },
+  { icon: '🔍', path: '/tools/regex',   title: 'Regex Tester',         desc: 'Reguläre Ausdrücke live testen' },
 ];
 
 function ToolOverview() {