fix(relay): send conversation history to SaaS relay (BUG-008)

SaaS Relay was sending only the current message without conversation
history, giving LLM no context from previous turns. Root cause:
streamStore passed only `content` string to chatStream(), and
saas-relay-client hard-coded a single-element messages array.

Fix:
- GatewayClient.chatStream() opts: add `history` field
- streamStore: extract last 20 messages as history before calling chatStream
- saas-relay-client: build messages array from history + current message

desktop/src/lib/gateway-client.ts

@@ -481,6 +481,8 @@ export class GatewayClient {
       reasoning_effort?: string;
       plan_mode?: boolean;
       subagent_enabled?: boolean;
+      /** Conversation history for relay clients that need full context */
+      history?: Array<{ role: string; content: string }>;
     }
   ): Promise<{ runId: string }> {
     const agentId = opts?.agentId || this.defaultAgentId;