v3.25.5: the PLI dictionary override guard fix

The live report: POST /api/pli-dict did not change the dictionary.
_PLI_DATA held a '' placeholder for every qualifier, and the clear
branch's pop removed the placeholder even when there was no override.
The PWA's Save posts every field, so one Save wiped the placeholders;
the guard `if code in _PLI_DATA` then silently skipped every later
override for those qualifiers (HTTP 200 + the unchanged/default dict)
until a restart - and the next Save wiped them again.

_PLI_DATA is now overrides-only ({}; the GET and _pli_data_hex already
fall back to PLI_DEFAULTS) and the guard tests PLI_QUALIFIER_NAMES, so a
new override always sticks and a clear never poisons a later set.

Tests: the endpoint test now saves/restores the dict (its pop(0x01)
cleanup was the same trap) plus a regression - the PWA-style full-dict
POST, then new overrides with and without defaults must stick, a clear
restores the default and a later set still works, an unknown qualifier
is ignored.  886 frontend + 874 Python; sw cache simple-v429.

frontend/index.html

@@ -1905,7 +1905,7 @@
 // ===== Version =====
 // Single source of truth for the PWA version: shown in the header and used
 // by the server version check in pysimConnect().
-const SIMPLE_VERSION = '3.25.4';
+const SIMPLE_VERSION = '3.25.5';
 document.getElementById('app-version').textContent = 'v' + SIMPLE_VERSION;
 
 // ===== view history (the browser Back/Forward buttons) =====