Fix model search spinner alignment

Target x-icon children in the model search button so the magnifier and loading spinner share the same grid cell. The stale span selector placed the spinner in a second row after the icon migration. Document the alignment contract.

Validation: 9 focused tests passed; verified centered icons during loading in the live WebUI at localhost:32081.

plugins/_model_config/AGENTS.md

@@ -58,6 +58,7 @@
 - Venice migration applies chat-only kwargs to chat slots and removes them from embedding slots without discarding embedding-specific kwargs.
 - On every startup after migration, short-circuit when `usr/plugins/_model_config/presets.yaml` exists. Only a missing collection may fetch `agent0ai/a0-presets`; parse remote and plugin-local fallback YAML through the same validator, strip secrets before persistence, and persist `mode_presets_fallback.yaml` when download or validation fails.
 - Model-name catalogs open below the input from either a field click or the embedded magnifier. Discard asynchronous results if their provider, API base, model draft, or query changed while the request was pending.
+- Keep the model-search magnifier and loading spinner in the same grid cell by targeting the button's `x-icon` children.
 - Additional parameter drafts retain plain-string compatibility but reject malformed JSON-shaped values and invalid KEY=VALUE lines with a standard notification. Reparse every preset slot before saving presets or API keys; invalid drafts stay editable and never silently save stale kwargs. Save errors identify the preset and model slot even when that draft is no longer selected.
 
 ## Work Guidance

plugins/_model_config/webui/model-field.html

@@ -376,7 +376,7 @@
   .model-search-btn:disabled {
     cursor: default;
   }
-  .model-search-btn > span {
+  .model-search-btn > x-icon {
     grid-area: 1 / 1;
     font-size: 18px;
     transition: opacity 0.15s;