refactor(search): fold zai-search into the glm provider

The separate zai-search entry showed "No connections" on the web search
page because credentials live on the `glm` connection, not on it. Every
other provider that does both chat and search (antigravity, kimi, xai,
gemini) declares webSearch on the provider itself, so do the same here.

- glm gains serviceKinds ["llm", "webSearch"] and the MCP searchConfig
- the request builder / normalizer move from "zai-search" to "glm"
- drop the zai-search registry entry and its svg logo, which also
  removes the only need for svg logo support in getProviderIconSrc

ollama-search keeps its own entry and credentialFallback: its search
endpoint is unrelated to the ollama chat transport.

src/shared/utils/providerIcon.js

@@ -8,9 +8,6 @@ const ICON_ALIASES = {
   "ollama-search": "ollama",
 };
 
-// Providers that ship an .svg logo instead of .png.
-const SVG_PROVIDER_IDS = new Set(["zai-search"]);
-
 // Runtime only — first 404 remembers id for the whole session
 const failedIds = new Set();
 
@@ -29,13 +26,10 @@ export function resolveProviderIconId(providerId) {
   return aliased;
 }
 
-/** `/providers/{id}.{png|svg}` or null when previously failed. */
+/** `/providers/{id}.png` or null when previously failed. */
 export function getProviderIconSrc(providerId) {
   const id = resolveProviderIconId(providerId);
-  if (!id) return null;
-  // svg for vector logos, png for everything else
-  const ext = SVG_PROVIDER_IDS.has(id) ? "svg" : "png";
-  return `/providers/${id}.${ext}`;
+  return id ? `/providers/${id}.png` : null;
 }
 
 /** Call from img onError so later mounts skip the request. */