Chat bubbles, icons for chats, more vivid and distinct colour tags

- Bubbles: own messages right-aligned in their own colour, agent answers left.
- Chat icons (22, validated on the server), shown in list, top bar, project page.
- Colour tags: eight fixed hues (new: pink) with theme-matched lightness and the
  highest chroma sRGB allows; stronger background tint.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

server/src/runner.rs

@@ -899,6 +899,7 @@ fn create_child(app: &Arc<App>, ctx: &ToolCtx, agent_name: &str, description: &s
         telegram_quiet: false,
         summary: String::new(),
         color: None,
+        icon: None,
     };
     let child = app.save_session(child)?;
     let msg = Message::new(Role::User, vec![Part::Text { text: prompt }]);