fix(relay): fix llm_routing read path bug and add User-Agent header for Coding Plan

1. connectionStore.ts: storedAccount.account.llm_routing → storedAccount.llm_routing
   - saveSaaSSession stores SaaSAccountInfo directly, not { account: SaaSAccountInfo }
   - This bug caused admin llm_routing config to never take effect

2. relay/service.rs: add User-Agent: claude-code/1.0 header
   - Kimi Coding Plan requires recognized coding agent User-Agent
   - Default reqwest UA is rejected with 403

3. Docs: add llm_routing routing mode explanation and troubleshooting entries

crates/zclaw-saas/src/relay/service.rs

@@ -243,6 +243,8 @@ pub async fn execute_relay(
 
         let mut req_builder = client.post(&url)
             .header("Content-Type", "application/json")
+            // Kimi Coding Plan 等 Coding Agent API 需要识别 User-Agent 为 coding agent
+            .header("User-Agent", "claude-code/1.0")
             .body(request_body.to_string());
 
         if let Some(ref key) = api_key {