scp81: always reuse the connection, collapsible options, label polish (v2.2.15)

- Keep-alive is no longer a setting: the TLS connection stays open between
  POSTs for the whole dialog (the card, as HTTP client, may still dial a new
  one at any time - GP Am. B 4.3.1) and only the 204 ends it, with a clean
  close_notify while the response is still buffered, then FIN. Dropped
  keep_alive/on_before_close/_scp81_wait_drained and the Connection-header
  'close' value (the API, status and UI no longer carry a keep-alive knob).
- The Listener Options block is a collapsed <details> with a 'custom' badge
  when anything differs from the reference defaults; the Reset button moved
  into the body so it cannot toggle the panel.
- X-Admin-Targeted-Application is opt-in (checkbox + //aid/... field, field
  disabled while off); X-Admin-Next-URI has a checkbox + hint explaining the
  one-shot rule of GP Am. B 4.4.2; both dependent fields grey out when
  unchecked.
- Labels/i18n: 'Chunked body (Transfer-Encoding: chunked)' stays English,
  'Show link events (...)', 'Теги comprehension-required'; the compact-header
  wording now spells out that it omits the optional space after ':' (legal
  per RFC 7230 3.2 OWS; saves one byte per header).
- Tests: a 200 keeps the socket for the next POST; the 204 closes with a
  mutual close_notify exchange; options helper/badge unit tests; removed the
  close-per-response and drain-wait tests. SW cache otaman-v181.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'otaman-v180';
+const CACHE = 'otaman-v181';
 const URLS = [
   'index.html',
   'help.html',