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/home.html

@@ -15,7 +15,7 @@
         </span>
         <span class="drop-title">Drop files to upload</span>
         <span class="drop-copy">or click to browse</span>
-        <span class="drop-meta">Max file size: {{.Data.MaxUploadSize}} · Links expire in 7 days</span>
+        <span class="drop-meta">{{if .Data.IsAdmin}}Admin upload: no file size limit{{else}}Max file size: {{.Data.MaxUploadSize}}{{end}} · Links expire in 7 days</span>
         <input id="file-input" name="file" type="file" multiple>
       </label>
 
@@ -25,6 +25,15 @@
           Advanced options
         </summary>
         <div class="option-grid">
+          {{if .CurrentUser}}
+          <label>
+            <span>Collection</span>
+            <select name="collection_id">
+              <option value="">Unsorted</option>
+              {{range .Data.Collections}}<option value="{{.ID}}">{{.Name}}</option>{{end}}
+            </select>
+          </label>
+          {{end}}
           <label>
             <span>Expires in</span>
             <select name="max_days">