Add a GitHub repo link to the nav (#35)

[aivanchenk]

Fixes #35.

Adds a GitHub icon-link to the nav so people can jump to the source repo.

What

• New GitHubIcon (matches the existing 16px icon set).
• IconButton gains an optional href prop: when set, it renders an <a target="_blank" rel="noreferrer noopener"> with the same styling instead of a <button>.
• NavActions renders the icon-link (label "View source on GitHub") pointing at https://github.com/EarthyScience/EarthPrints, placed before the About dropdown and theme toggle.

Verify

• eslint clean on the changed files.
• tsc --noEmit shows only pre-existing errors in mapLabels.test.ts, none from this change.

[gemini-code-assist]

Code Review

This pull request adds a GitHub link to the navigation actions. To support this, the IconButton component is updated to render an anchor (<a>) tag when an href prop is provided, and a new GitHubIcon component is introduced. The review feedback correctly points out that when href is provided, any passed onClick handler is ignored, and suggests forwarding the onClick prop to the anchor tag.

Important

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[github-actions]

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