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

ident_switch.php

@@ -42,12 +42,18 @@ class ident_switch extends rcube_plugin
 	/** @var int SMTP authentication: no authentication required. */
 	public const SMTP_AUTH_NONE = 2;
 
+	/** @var int SMTP authentication: use custom credentials. */
+	public const SMTP_AUTH_CUSTOM = 3;
+
 	/** @var int Sieve authentication: use same credentials as IMAP. */
 	public const SIEVE_AUTH_IMAP = 1;
 
 	/** @var int Sieve authentication: no authentication required. */
 	public const SIEVE_AUTH_NONE = 2;
 
+	/** @var int Sieve authentication: use custom credentials. */
+	public const SIEVE_AUTH_CUSTOM = 3;
+
 	/** @var int Notification checking: enabled. */
 	public const NOTIFY_CHECK_ENABLED = 1;
 
@@ -189,7 +195,7 @@ class ident_switch extends rcube_plugin
 		while ($r = $rc->db->fetch_assoc($qRec)) {
 			$accValues[] = $r['id'];
 			$iidMap[$r['iid']] = $r['id'];
-			if ($iid == $r['iid']) {
+			if ($iid === (int)$r['iid']) {
 				$accSelected = $r['id'];
 			}