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/admin.html

@@ -1,16 +1,24 @@
 {{define "admin.html"}}{{template "base" .}}{{end}}
 
 {{define "content"}}
-<section class="admin-view" aria-labelledby="admin-title">
+<section class="app-shell admin-shell" aria-labelledby="admin-title">
+  <aside class="app-sidebar">
+    <a class="sidebar-link {{if eq .Data.Section "overview"}}is-active{{end}}" href="/admin">Overview</a>
+    <a class="sidebar-link {{if eq .Data.Section "files"}}is-active{{end}}" href="/admin/files">Files</a>
+    <a class="sidebar-link" href="/admin/users">Users</a>
+    <a class="sidebar-link" href="/admin/settings">Settings</a>
+    <a class="sidebar-link" href="/app">My files</a>
+    <form class="sidebar-logout" action="/admin/logout" method="post">
+      <button class="button button-outline" type="submit">Logout</button>
+    </form>
+  </aside>
+
+  <div class="app-main">
   <div class="admin-header">
     <div>
       <p class="kicker">Operator console</p>
-      <h1 id="admin-title">Admin overview</h1>
+      <h1 id="admin-title">{{.Data.PageTitle}}</h1>
     </div>
-    <form action="/admin/logout" method="post">
-      <a class="button button-outline" href="/admin/users">Users</a>
-      <button class="button button-outline" type="submit">Logout</button>
-    </form>
   </div>
 
   <div class="metric-grid">
@@ -94,5 +102,6 @@
       </div>
     </div>
   </div>
+  </div>
 </section>
 {{end}}