feat: enhance regex tester with interactive playground features

Summary

Transforms the basic regex tester into an interactive playground inspired by regexer. The enhancement adds several features to make regex more accessible to junior developers:

Core Features:

• Toggleable pattern flags (g, i, m, s, u, y) with hover card explanations
• Preset regex patterns (email, URL, phone, date, IP, hex color, HTML tag, credit card) for quick demos
• Real-time pattern explanation with color-coded components (hover for details)
• Capture group visualization in CSS box model style with nested indentation (collapsible section)
• Match statistics dashboard showing total/unique matches, average length, and execution time
• Collapsible regex cheat sheet with click-to-insert functionality
• Enhanced match highlighting with hover cards showing position and length

Technical Changes:

• Added 6 new React components in components/regex/
• Extended regex-tester.utils.ts with pattern parsing, explanation, and stats functions
• Added @radix-ui/react-hover-card and @radix-ui/react-tooltip dependencies via shadcn
• Updated main page to use tabs for Pattern Explanation and Statistics

Layout & UX Improvements:

• Widened layout from max-w-2xl to max-w-4xl for more breathing room
• Moved capture groups to a collapsible section in the main card for easier access
• Replaced Radix Tooltip with HoverCard (fixes content not updating when moving mouse quickly)
• All hover cards use sans-serif fonts consistently with bg-muted background highlighting for code values
• Email preset includes gm flags for multi-line matching
• Clear button resets all fields

Review & Testing Checklist for Human

• Test capture groups collapsible section: Enter a pattern with groups like /(a)(b)/g with test string ab ab, expand the "Capture Groups" section, verify visualization displays correctly
• Test preset patterns: Click through presets (especially Email), verify they load correctly with proper flags and show matches
• Verify font consistency: Hover over flags, pattern components, capture groups, and match highlights - all should use sans-serif fonts with background highlighting for code values
• Test clear button: Fill in pattern and test string, click Clear, verify all fields reset including flags

Recommended test plan:

1. Load page fresh, select Email preset - verify it matches emails in the test string
2. Enter a pattern with capture groups, expand the collapsible section, verify groups display
3. Hover over various elements (flags, pattern components, matches) and verify consistent styling
4. Click Clear and verify everything resets
5. Check the wider layout looks good on different screen sizes

Notes

• Capture groups are now in a collapsible section in the main card rather than a separate tab
• The tabs section contains only Pattern Explanation and Statistics

Link to Devin run: https://app.devin.ai/sessions/93dfd4f1aa4649e7917bfc1f71f62694
Requested by: @peckz