feat: add repository and deployment links (#84)

## 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

Author: ShoGinn

README.md

@@ -7,6 +7,8 @@
 
 A local-first federal tax calculator and W-4 optimizer for modeling income, reconciling withholding, and planning the rest of the tax year.
 
+**[Open Tax Tracker on Railway](https://shoginn-tax-tracker.up.railway.app)**
+
 Tax Tracker combines a React interface with a stateless FastAPI calculation service. Personal records and settings stay in the browser; the API receives calculation inputs only for the duration of each request and does not persist them.
 
 > [!IMPORTANT]

frontend/src/app/AppShell.test.tsx

@@ -17,6 +17,10 @@ describe("AppShell", () => {
     expect(screen.getByRole("link", { name: "Overview" })).toBeInTheDocument();
     expect(screen.getByRole("link", { name: "Income" })).toBeInTheDocument();
     expect(screen.getByRole("link", { name: "Tax position" })).toBeInTheDocument();
+    expect(screen.getByRole("link", { name: "GitHub" })).toHaveAttribute(
+      "href",
+      "https://github.com/ShoGinn/tax-tracker",
+    );
     expect(screen.getByRole("heading", { name: "Your federal tax plan" })).toBeInTheDocument();
   });
 });

frontend/src/app/AppShell.tsx

@@ -13,6 +13,8 @@ const manageNavigation = [
   { to: "/settings", label: "Profile & backup", marker: "•" },
 ];
 
+const githubUrl = "https://github.com/ShoGinn/tax-tracker";
+
 const pageMeta: Record<string, { eyebrow: string; title: string; description: string }> = {
   "/": {
     eyebrow: "Plan overview",
@@ -96,6 +98,16 @@ export const AppShell = () => {
           <NavigationList items={planNavigation} />
           <p className="nav-section-label nav-section-label--spaced">Manage</p>
           <NavigationList items={manageNavigation} />
+          <ul>
+            <li>
+              <a className="nav-link" href={githubUrl} target="_blank" rel="noopener noreferrer">
+                <span className="nav-marker" aria-hidden="true">
+                  GH
+                </span>
+                <span>GitHub</span>
+              </a>
+            </li>
+          </ul>
         </nav>
 
         <div className="sidebar-note">