feat: create browser checklist

[yvl4life]

Frontend browser compatibility implemented

What changed

• Added BROWSER_COMPATIBILITY.md

  • documents supported browsers
  • records reported browser-specific issues
  • describes fixes and test matrix

• Updated package.json

  • added browserslist for Chrome, Firefox, Safari, Edge, iOS, Android

• Updated playwright.config.ts

  • added Playwright projects for:
    • chromium
    • firefox
    • webkit
    • mobile-chrome
    • mobile-safari
    • edge

• Added browser helper in browser.ts

  • copyTextToClipboard with fallback
  • supportsMatchMedia safe detection

• Fixed compatibility code

  • SocialShareButtons.tsx
    • uses clipboard fallback helper
  • page.tsx
    • uses Web Share API fallback
    • falls back to copy/prompt for legacy browsers
  • ThemeToggle.tsx
    • safe window.matchMedia handling
    • localStorage access wrapped in try/catch

• Added tests

  • browser-compatibility.test.ts

Validation

• Checked changed files for errors: no issues found

Closes #601

Made changes.

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