fix: address CodeRabbit findings on the style-editor/font-selector PR

- Fix _load_font_sized double-wrapping the (font, size) tuple on the
  missing-font path, which handed callers a tuple instead of a font.
- Fix _set_nested_value skipping an explicit None when the key already
  existed, which silently kept stale overrides when a user cleared a
  nullable per-mode field or blanked all channels of an indexed color.
- Preserve BDF scalable/native_size metadata through fetchFontCatalog's
  catalog-format mapping so maxFixedSize filtering actually applies.
- Stop caching an empty array on a failed font-catalog fetch so a later
  call can retry instead of being stuck with the failed result.
- Keep a saved font selected in the style editor even when it no longer
  fits a newly declared maxFixedSize, instead of silently deselecting it.
- Don't drop in-progress user edits to fallback fields when a plugin
  widget finishes loading asynchronously and takes over the form.
- Tighten the removed font-override endpoint test to assert 405, not
  just != 200.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web_interface/blueprints/api_v3/__init__.py

@@ -1026,9 +1026,11 @@ def _set_nested_value(config, key_path, value):
             current[seg] = {}
         current = current[seg]
 
-    # Set the final value (don't overwrite with empty dict if value is None and we want to preserve structure)
-    if value is not None or segments[-1] not in current:
-        current[segments[-1]] = value
+    # Set the final value. _SKIP_FIELD (checked above) is the only sentinel
+    # for "leave the existing value alone" -- an explicit None here is a real
+    # value (e.g. the per-mode "inherit the base" override) and must overwrite
+    # whatever was already stored.
+    current[segments[-1]] = value
 def _set_missing_booleans_to_false(config, schema_props, form_keys, prefix='', config_node=None):
     """Walk schema and set missing boolean form fields to False.