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/store/chat/streamStore.ts

@@ -296,6 +296,13 @@ export const useStreamStore = create<StreamState>()(
         useConversationStore.setState({ sessionKey: effectiveSessionKey });
       }
 
+      // Build conversation history for relay clients (last 20 messages ≈ 10 turns)
+      const history = (_chat?.getMessages() || [])
+        .filter(m => m.role === 'user' || m.role === 'assistant')
+        .filter(m => !m.streaming && !m.optimistic)
+        .map(m => ({ role: m.role, content: m.content }))
+        .slice(-20);
+
       const result = await client.chatStream(
         content,
         {
@@ -528,6 +535,7 @@ export const useStreamStore = create<StreamState>()(
           reasoning_effort: get().getChatModeConfig().reasoning_effort,
           plan_mode: get().getChatModeConfig().plan_mode,
           subagent_enabled: get().getChatModeConfig().subagent_enabled,
+          history,
         }
       );