Add custom auth, connection testing, preconfig_only, and security fixes

Major form improvements:
- Custom SMTP/Sieve credentials (separate username/password per protocol)
- Connection testing on save (IMAP, SMTP, Sieve) with localized errors
- preconfig_only mode to restrict domains to preconfigured entries
- Form POST value preservation on save errors (auth selects, passwords)
- Smart host placeholders (SMTP/Sieve default to IMAP host)

Security and bug:
- Fix password re-encryption bug (was comparing raw vs encrypted values)
- Fix XSS: escape label output in special folders form
- Fix parse_url() return value not checked for false
- Fix decrypt() failures not handled (fallback to empty string)
- Sanitize log output (remove raw POST data from log messages)
- Replace weak == comparisons with strict === (PHP and JS)

SQL changes:
- Consolidate 4 migrations (2026021000-03) into single 2026021000
- Remove now unused notify_sound_url column
- Add smtp_username, smtp_password, sieve_username, sieve_password columns

config.inc.php.dist

@@ -82,3 +82,13 @@ $config['ident_switch.round_robin'] = false;
  * Default: false (warning is displayed).
  */
 $config['ident_switch.hide_notifier_warning'] = false;
+
+/*
+ * Restrict account switching to preconfigured domains only.
+ * When enabled, the ident_switch form is hidden for identities whose email
+ * domain does not match any entry in 'ident_switch.preconfig'.
+ * Users can still create Roundcube identities (name, signature, etc.) but
+ * cannot configure server connections for non-preconfigured domains.
+ * Default: false (all domains allowed).
+ */
+$config['ident_switch.preconfig_only'] = false;