design: backup-reauth - the account password before a backup or an export

Agreed with the owner on 2026-10-13 (roadmap backup-reauth), taken up on
2026-10-16 ("Да"). The Full backup form on Server › Backup and the Export
domain form on a domain's settings get one field each, "Your account
password" (autocomplete current-password, required), before the button,
saying why it is asked at every download. The help's Backup topic says both
ask for it and that wrong guesses count against the password change's
limit. check.py: 0 violations; outlines unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/assets/panel-redesign/panel/backup.html

@@ -86,6 +86,8 @@
                 <div class="field"><label class="label" for="pw2">Repeat password</label><div class="control"><input class="input" id="pw2" type="password"></div></div>
               </div>
               <p class="help mb-3">Keep this password: SelfPost does not store it, and the file cannot be opened without it.</p>
+              <div class="field"><label class="label" for="acpw">Your account password</label><div class="control"><input class="input" id="acpw" name="account_password" type="password" autocomplete="current-password" required></div>
+                <p class="help">The one you sign in with, asked at every download: the archive holds every secret of the server, and a session left open must not be enough to take it.</p></div>
               <button type="submit" class="button is-primary"><i class="ti ti-download"></i>Download full backup</button>
             </form>
           </div>