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/dashboard.html

@@ -0,0 +1,70 @@
+{{define "dashboard.html"}}{{template "base" .}}{{end}}
+
+{{define "content"}}
+<section class="app-shell" aria-labelledby="dashboard-title">
+  <aside class="app-sidebar">
+    <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="collection-create" action="/app/collections" method="post">
+      <label>
+        <span>New collection</span>
+        <input name="name" placeholder="Projects">
+      </label>
+      <button class="button button-outline" type="submit">Create</button>
+    </form>
+  </aside>
+
+  <div class="app-main">
+    <div class="admin-header">
+      <div>
+        <p class="kicker">Personal space</p>
+        <h1 id="dashboard-title">My files</h1>
+        <p class="muted-copy">{{.Data.StorageUsed}} used · max file size {{.Data.MaxUploadSize}}</p>
+      </div>
+      <a class="button button-primary" href="/">Upload files</a>
+    </div>
+
+    <div class="collection-tabs">
+      <a class="button {{if not .Data.Selected}}button-primary{{else}}button-outline{{end}}" href="/app">All</a>
+      {{range .Data.Collections}}
+        <a class="button {{if eq $.Data.Selected .ID}}button-primary{{else}}button-outline{{end}}" href="/app?collection={{.ID}}">{{.Name}}</a>
+      {{end}}
+    </div>
+
+    <div class="card admin-table-card">
+      <div class="card-content">
+        <div class="table-header"><h2>Owned boxes</h2><p>Collections organize boxes. Shared links remain unlisted.</p></div>
+        <div class="admin-table-wrap">
+          <table class="admin-table">
+            <thead><tr><th>Title</th><th>Collection</th><th>Files</th><th>Size</th><th>Created</th><th>Expires</th><th>Actions</th></tr></thead>
+            <tbody>
+              {{range .Data.Boxes}}
+                <tr>
+                  <td class="file-name">{{.Title}}</td>
+                  <td>{{if .CollectionName}}{{.CollectionName}}{{else}}Unsorted{{end}}</td>
+                  <td>{{.FileCount}}</td>
+                  <td>{{.Size}}</td>
+                  <td>{{.CreatedAt}}</td>
+                  <td>{{.ExpiresAt}}</td>
+                  <td class="table-actions">
+                    <a class="button button-outline" href="{{.URL}}" target="_blank" rel="noopener noreferrer">Open</a>
+                    <form action="/app/boxes/{{.ID}}/rename" method="post"><input class="compact-input" name="title" placeholder="Rename"><button class="button button-outline" type="submit">Save</button></form>
+                    <form action="/app/boxes/{{.ID}}/move" method="post">
+                      <select name="collection_id"><option value="">Unsorted</option>{{range $.Data.Collections}}<option value="{{.ID}}">{{.Name}}</option>{{end}}</select>
+                      <button class="button button-outline" type="submit">Move</button>
+                    </form>
+                    <form action="/app/boxes/{{.ID}}/delete" method="post"><button class="button button-danger" type="submit">Delete</button></form>
+                  </td>
+                </tr>
+              {{else}}
+                <tr><td colspan="7">You have no boxes yet.</td></tr>
+              {{end}}
+            </tbody>
+          </table>
+        </div>
+      </div>
+    </div>
+  </div>
+</section>
+{{end}}