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

SQL/sqlite/2026021000.sql

@@ -1,3 +1,5 @@
+-- Upgrade from v4.x to v5.x
+-- SQLite: recreate table with all new columns
 PRAGMA foreign_keys=off;
 BEGIN TRANSACTION;
 
@@ -43,6 +45,36 @@ CREATE TABLE ident_switch
 		smallint
 		NOT NULL
 		DEFAULT 1,
+	smtp_username
+		varchar(64),
+	smtp_password
+		varchar(255),
+	sieve_host
+		varchar(64),
+	sieve_port
+		integer
+		CHECK(sieve_port > 0 AND sieve_port <= 65535),
+	sieve_auth
+		smallint
+		NOT NULL
+		DEFAULT 1,
+	sieve_username
+		varchar(64),
+	sieve_password
+		varchar(255),
+	notify_check
+		smallint
+		NOT NULL
+		DEFAULT 1,
+	notify_basic
+		smallint
+		DEFAULT NULL,
+	notify_sound
+		smallint
+		DEFAULT NULL,
+	notify_desktop
+		smallint
+		DEFAULT NULL,
 	drafts_mbox
 		varchar(64),
 	sent_mbox