feat: add upload policies, daily limits, and storage quotas

- Add environment variables to configure anonymous uploads, daily upload caps, and default user storage limits.
- Update config loader to parse and validate the new settings.
- Implement backend logic to track daily usage and active storage per user.
- Update README and `.env.example` to document the new settings and admin panels.

backend/templates/pages/dashboard.html

@@ -6,6 +6,9 @@
     <a class="sidebar-link is-active" href="/app">Dashboard</a>
     <a class="sidebar-link" href="/account/settings">Settings</a>
     {{if eq .Data.User.Role "admin"}}<a class="sidebar-link" href="/admin">Admin</a>{{end}}
+    <form class="sidebar-logout" action="/logout" method="post">
+      <button class="button button-outline" type="submit">Logout</button>
+    </form>
     <form class="collection-create" action="/app/collections" method="post">
       <label>
         <span>New collection</span>