feat(web): let schemas label enum dropdown options

An enum property renders as a dropdown whose option text is derived from
the value — underscores replaced, title case applied. That works when the
value reads as its own label and fails when it does not: "vs" renders as
"Vs", and "abbrev" tells the user nothing about the "Sep 19" it produces.
Schemas had no way to say otherwise, so the label was whatever the config
key happened to look like.

Enum dropdowns now take their option text from x-options.labels when the
schema supplies it. This is not a new convention: the checkbox-group
widget has read x-options.labels since it was written, with the same
humanised fallback. This extends it to plain enums and to array-table
columns.

Display only — the option value, and so the saved config, is unchanged.
The map may be partial; unlabelled values keep the humanised fallback, so
every existing schema renders exactly as before. Older cores ignore
x-options entirely, which means a plugin can ship labels without
requiring users to upgrade first.

Array-table columns get the same lookup but keep the raw value as their
fallback rather than the humanised one. Those columns hold values such as
ticker symbols, where "aapl" -> "Aapl" would be wrong, and they were not
being humanised before this change.

Verified against the running web service: with labels the hockey plugin's
date dropdown reads "Sep 19 / 9/19 / 19 Sep / 19/9 / Fri Sep 19"; with
the pre-change template and the same schema it falls back to
"Abbrev / Numeric / Day First / ...", confirming the degradation path.

.github/workflows/test.yml

@@ -85,5 +85,6 @@ jobs:
             test/test_plugin_update_reservation.py \
             test/test_template_targets.py \
             test/test_widget_scripts.py \
+            test/test_enum_option_labels.py \
             test/test_doc_links.py \
             test/web_interface/test_cache.py