feat(accounts): implement user accounts, sessions, and dashboards

Introduce Stage 4 features to support multi-user accounts, cookie-based web sessions, and personal dashboards.

Changes include:
- Adding `/register` to bootstrap the first admin account and `/login`/`/logout` for session management.
- Creating a personal dashboard (`/app`) to display owned boxes, storage usage, and upload history.
- Implementing admin user management (`/admin/users`) for generating invite links and managing user states.
- Updating the bbolt database schema to store users, sessions, invites, and collections.
- Adding `golang.org/x/crypto` for password hashing and introducing unit tests for account handlers.

backend/templates/layouts/base.html

@@ -27,6 +27,13 @@
         <span>{{.AppName}}</span>
       </a>
       <div class="nav-links">
+        {{if .CurrentUser}}
+        <a class="button button-ghost" href="/app">My files</a>
+        <a class="button button-ghost" href="/account/settings">Account</a>
+        <form action="/logout" method="post" class="inline-form"><button class="button button-outline" type="submit">Logout</button></form>
+        {{else}}
+        <a class="button button-ghost" href="/login">Login</a>
+        {{end}}
         <a class="button button-ghost" href="/api">API</a>
         <a class="button button-outline" href="/healthz">Health</a>
       </div>
@@ -39,7 +46,7 @@
 
   <footer class="site-footer">
     <span>{{.AppName}} · {{.CurrentYear}} · self-hosted</span>
-    <span class="footer-links"><a href="/">Upload</a><a href="/healthz">Health</a></span>
+    <span class="footer-links"><a href="/">Upload</a>{{if .CurrentUser}}<a href="/app">My files</a>{{end}}<a href="/healthz">Health</a></span>
   </footer>
 </body>
 </html>