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.

public/providers/zai-search.svg

@@ -1,5 +0,0 @@
-<svg width="128" height="128" viewBox="0 0 128 128" fill="none" xmlns="http://www.w3.org/2000/svg">
-  <rect width="128" height="128" rx="28" fill="#6366f1"/>
-  <text x="64" y="64" text-anchor="middle" dominant-baseline="central" font-family="system-ui,-apple-system,sans-serif" font-size="48" font-weight="700" fill="#ffffff">Z</text>
-  <text x="64" y="98" text-anchor="middle" dominant-baseline="central" font-family="system-ui,-apple-system,sans-serif" font-size="16" font-weight="600" fill="#c7d2fe">.AI</text>
-</svg>