Add repository and deployment links

[ShoGinn]

Summary

• add a GitHub repository link to the app navigation
• add the live Railway deployment URL to the README
• cover the new navigation link in the app shell test

Verification

• frontend tests
• frontend typecheck
• frontend lint and formatting
• pre-commit and pre-push hooks

[Copilot]

Pull request overview

Adds discovery links for the project by surfacing the repository in the app’s sidebar navigation and publishing the live Railway deployment URL in the README, with a small AppShell test update to cover the new navigation link.

Changes:

• Add a prominent Railway deployment link near the top of README.md.
• Add a GitHub repository link to the AppShell sidebar navigation.
• Extend the AppShell test to assert the GitHub link and its href.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
README.md	Adds a top-level link to the live Railway deployment.
frontend/src/app/AppShell.tsx	Adds a GitHub repository link to the sidebar navigation.
frontend/src/app/AppShell.test.tsx	Adds a test assertion for the new GitHub navigation link.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Suppressed comments (3)

frontend/src/app/AppShell.tsx:103

• This link opens a new tab via target="_blank" but the accessible name doesn’t indicate that behavior. Consider adding an aria-label that includes “opens in a new tab” so screen-reader users aren’t surprised.

              <a className="nav-link" href={githubUrl} target="_blank" rel="noopener noreferrer">

README.md:11

• The README now presents a one-click link to the public deployment before the existing warning about not entering real financial data into untrusted deployments. To reduce the chance users miss the warning, consider adding a short caution immediately adjacent to the deployment link.

**[Open Tax Tracker on Railway](https://shoginn-tax-tracker.up.railway.app)**

frontend/src/app/AppShell.test.tsx:23

• If the GitHub link’s accessible name is updated to mention opening a new tab, this test should query by the new accessible name to match user-facing semantics.

    expect(screen.getByRole("link", { name: "GitHub" })).toHaveAttribute(
      "href",
      "https://github.com/ShoGinn/tax-tracker",
    );