feat: show live GitHub star count in navbar

[bilal-rauf-dev]

Closes #370

What this does

Replaces the dead "⭐ Star on GitHub" button in the navbar (it had no
href/onClick) with a real link to the repo that also shows the current
star count.

Before: static text, not clickable
After: clickable link → "⭐ 123 Star on GitHub" (formatted as "1.2k" for ≥1000)

Note: the count is fetched once on page load, not polled — "live" here
means "current as of this page load," not continuously updating.

Implementation

Followed the existing pattern in useSuggestTags.jsx of keeping fetch
logic in a dedicated hook rather than inline in the component.

New — src/hooks/useGithubStars.js

• Takes an optional repo param (default "anoopcodehack/DevBoard")
• Fetches stargazers_count via axios + async/await on mount
• Swallows errors (including GitHub's unauthenticated 403 rate limit) —
  logs a console.warn, leaves stars as null, never throws

Modified — src/pages/Dashboard.jsx

• Uses useGithubStars(), adds a small formatStars helper
• Swaps the <button> for an <a> (target="_blank" rel="noopener noreferrer")
• Falls back to plain "⭐ Star on GitHub" while loading or on fetch failure —
  no layout shift
• Reuses the existing Tailwind classes from the sibling "🗑️ Clear Done"
  button — no new styles

Testing done

• npm run dev in client/, confirmed the link renders immediately and
  updates once the count loads
• Confirmed it's clickable and opens the repo in a new tab
• Blocked api.github.com in devtools to simulate a rate limit / offline
  case — button correctly falls back to plain text, no console errors,
  no crash

Checklist

• My code follows the project's style guidelines
• I tested my changes locally
• I updated the README (not needed — no setup/usage change)
• My commit messages are clear and follow the conventional commit format