fix: add system prompt to chat TUI to prevent non-English output

The chat command wasn't sending a system prompt with messages, unlike the
run command which always prepends "You are a helpful AI assistant." Small
bilingual models like Qwen3.5-0.8B would default to Chinese without an
English system prompt to anchor the language.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: evilsocket

cake-cli/src/chat.rs

@@ -135,7 +135,10 @@ impl App {
     fn new(server: &str) -> Self {
         Self {
             tab: Tab::Chat,
-            messages: Vec::new(),
+            messages: vec![ChatMessage {
+                role: "system".to_string(),
+                content: "You are a helpful AI assistant.".to_string(),
+            }],
             input: String::new(),
             cursor_pos: 0,
             scroll_offset: 0,
@@ -195,6 +198,10 @@ fn build_message_lines(app: &App, width: u16) -> Vec<Line<'static>> {
             lines.push(Line::from(""));
         }
 
+        if msg.role == "system" {
+            continue;
+        }
+
         let is_last = i == app.messages.len() - 1;
 
         if msg.role == "user" {