Drop fabricated SET UP IDLE MODE TEXT qualifier decode; bump to 1.15.1

Spec verification against UICC_SPECS.md / TS 102 223 V18.3.0 §8.6
showed the Command Qualifier for SET UP IDLE MODE TEXT is RFU
("this byte is RFU") — no icon self-explanatory bit exists in any
generation (GSM 11.14 predates the command; TS 31.111 does not
override it). The v1.11.0 _idle_text_qualifier decoded bit 1 as
"icon self-explanatory" from an unverified source.

Remove the decoder; the command now uses generic RFU handling
(hidden at '00', raw hex otherwise). Tests updated accordingly.
Version 1.15.1, SW cache v21.

frontend/index.html

@@ -1881,7 +1881,7 @@ function onFilterChange() {
 }
 
 // ──────────────────── Versioning ────────────────────
-const PWA_VERSION = '1.15.0';
+const PWA_VERSION = '1.15.1';
 const MIN_SERVER_VERSION = '1.0.0';
 
 function parseVersion(v) {