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/pages/admin.html

@@ -8,6 +8,7 @@
       <h1 id="admin-title">Admin overview</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>
@@ -54,6 +55,7 @@
           <thead>
             <tr>
               <th>Box</th>
+              <th>Owner</th>
               <th>Files</th>
               <th>Size</th>
               <th>Downloads</th>
@@ -67,6 +69,7 @@
             {{range .Data.Boxes}}
               <tr>
                 <td><code>{{.ID}}</code></td>
+                <td>{{.Owner}}</td>
                 <td>{{.FileCount}}</td>
                 <td>{{.TotalSizeLabel}}</td>
                 <td>{{.DownloadCount}}{{if .MaxDownloads}} / {{.MaxDownloads}}{{end}}</td>
@@ -84,7 +87,7 @@
                 </td>
               </tr>
             {{else}}
-              <tr><td colspan="8">No uploads yet.</td></tr>
+              <tr><td colspan="9">No uploads yet.</td></tr>
             {{end}}
           </tbody>
         </table>