feat: the remaining six event types get their parameter forms

MT call, Call connected, Call disconnected, Browser termination, (I-)WLAN
access status and CSG cell selection no longer fall back to the generic
"send a bare notification" prompt:

- MT call (TS 31.111 7.5.1A.2): transaction identifier, calling party number
  + TON -> Address (EFADN BCD coding, encBcdDigits/eventAddressTlv),
  subaddress, IMS URI, media type; source network.
- Call connected (7.5.2A.2) / Call disconnected (7.5.3A.2): transaction
  identifier, near/far-end source select, media type, and a cause - absent,
  the zero-length radio-link-timeout form or custom bytes (the hex field shows
  only for "custom").
- Browser termination (TS 102 223 7.5.9.2): user / error cause.
- (I-)WLAN access status (TS 31.111 7.5.1.2): no coverage / coverage /
  connection on-going.
- CSG cell selection (TS 31.111 7.5.3.2): status + selection mechanism,
  optional access technology, CSG ID, HNB name and PLMN ID; source network.

The device-identities source (TS 102 223 8.7: 82 terminal / 83 network) is now
selectable: EVENT_FORMS carries a fixed `src` or a `srcField`, and
_send_event_download()/`/api/event-send` take an `event_src` override (an
invalid value falls back to the terminal).

Tests: the BCD/Address builders, each event's build vectors, the "no not_yet
left" guard and the server source override.  Help EN/RU, both AGENTS files;
sw cache simple-v348.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simple-v347';
+const CACHE = 'simple-v348';
 const URLS = [
   'index.html',
   'help.html',