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

Agreed with the owner on 2026-10-13, taken up on 2026-10-16 ("Да"); the
mockup is 1888dce. Both secret downloads - the full backup on Server ›
Backup and a domain's export on its settings - ask for "Your account
password": reauth (handlers_backup.go) checks the signed-in user's own
password with bcrypt, as a password change does, under the same limiter
(auth.AllowPasswordChange: ten in fifteen minutes per user, shared with the
Account form). A wrong or missing one is 401 on the same page with "Your
account password is incorrect.", logged; nothing is sent. The import and
the selfpost-backup CLI are not asked.

Also, the owner's phone screenshot of 2026-10-16 ("кнопка выбора файла не
такая, как в макете. Это косяк?"): the import's file field was a bare
<input type="file"> drawn by the browser ("Browse…"), where the mockup has
Bulma's .file - "Choose a file…" and the name chosen. The template now
follows the mockup; panel.js writes the name into .file-name on change and
at load (testdata/filename_test.js, run by TestPanelJS). No other file
field exists.

Tests: TestSecretDownloadsAskForTheAccountPassword (missing, wrong, right,
the limiter), the export refusals of the settings page with the password,
cmd/panel's restore tests and e2e's backups with it, e2e's refusal of a
backup without and with a wrong password. go vet + go test (26 packages)
and make e2e on the test server for this tree.

Docs: security.md § Backup and domain export, guide § Full backup and
restore, help (word for word with the mockup); CHANGELOG [Unreleased];
roadmap row done.

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

cmd/panel/restore_test.go

@@ -360,10 +360,11 @@ func signIn(t *testing.T, h http.Handler) *http.Cookie {
 
 // downloadBackup takes a backup through the panel's own /backup route, the way
 // the operator does. An empty password downloads the plain archive; otherwise
-// the response is a .spbk envelope, which is decrypted here.
+// the response is a .spbk envelope, which is decrypted here. The account
+// password goes with it, as the form asks for it (backup-reauth).
 func downloadBackup(t *testing.T, h http.Handler, session *http.Cookie, password string) []byte {
 	t.Helper()
-	form := url.Values{}
+	form := url.Values{"account_password": {restorePassword}}
 	if password != "" {
 		form.Set("encrypt", "1")
 		form.Set("password", password)