fix: parse templates individually to prevent content block overwrite

Root cause: ParseFS with wildcard html/*.html caused all {{define "content"}}

blocks to overwrite each other — last alphabetically (register.html) won for

all pages. Now each page is parsed separately with base.html as its own template.

Also fix footer link: GitHub → Gitea project page.

internal/templates/html/base.html

@@ -348,7 +348,7 @@
     </header>
     <main><div class="container">{{template "content" .}}</div></main>
     <footer>
-        <p>MrixsCraft Server &copy; 2026 · <a href="https://github.com/Mrixs/MrixsCraft-server">GitHub</a></p>
+        <p>MrixsCraft Server &copy; 2026 · <a href="https://gitea.mrixs.me/Mrixs/MrixsCraft">Gitea</a></p>
     </footer>
     <script>
     // Update nav based on auth state