refactor: clean up themes, templates, and add comprehensive README

- Themes now only override CSS variables (no selector overrides)
- Removed duplicate styles from header.tmpl (just loads Inter font now)
- Cleaned up logo SVG (simpler, no overlapping text)
- Improved home page with proper hero layout and sign-in/explore buttons
- Added detailed README documenting the setup, deployment, and design principles

templates/custom/footer.tmpl

@@ -1,12 +1,10 @@
-<footer style="background: var(--color-body-secondary, #f8fafc); border-top: 1px solid var(--color-border, #e2e8f0); padding: 2rem 0; margin-top: 3rem; text-align: center; color: var(--color-text-secondary, #475569);">
-  <div style="max-width: 1200px; margin: 0 auto; padding: 0 1rem;">
-    <p style="margin: 0.5rem 0; font-size: 0.95rem;">
-      <strong style="color: var(--color-text, #1e293b); font-weight: 600;">mGit</strong> - A Git hosting solution
-    </p>
-    <p style="margin: 0.5rem 0; font-size: 0.9rem;">
-      Powered by <a href="https://gitea.io" target="_blank" rel="noopener" style="color: var(--color-link, #2563eb); text-decoration: none;">Gitea</a>
-      <span style="margin: 0 0.5rem;">•</span>
-      <a href="https://flexsiebels.de" target="_blank" rel="noopener" style="color: var(--color-link, #2563eb); text-decoration: none;">flexsiebels.de</a>
-    </p>
-  </div>
-</footer>
+<script>
+  // Replace default footer text
+  (function() {
+    const footer = document.querySelector('.page-footer');
+    if (footer) {
+      const powered = footer.querySelector('.powered-by, [class*="powered"]');
+      if (powered) powered.innerHTML = 'Powered by <a href="https://gitea.io" target="_blank">Gitea</a>';
+    }
+  })();
+</script>