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/__init__.py

@@ -5,3 +5,8 @@ from .jwtdecoder import jwt_decoder_blueprint
 from .passwordgen import passwordgen_blueprint
 from .timestamp import timestamp_blueprint
 from .textdiff import textdiff_blueprint
+from .qrcode_gen import qrcode_blueprint
+from .markdown_tool import markdown_blueprint
+from .colorconverter import color_blueprint
+from .jsonformatter import json_formatter_blueprint
+from .regextester import regex_blueprint