feat: Enhance token usage tracking and improve chat UI with emojis

src/app/subagent/engine.rs

@@ -449,12 +449,22 @@ pub fn run_subagent(ctx: &SubagentContext, tx: &mpsc::Sender<SubagentEvent>) ->
         // drain thread can accumulate it and update the Usage panel.
         // Without this, the Usage panel always shows zeros because the
         // subagent never tells the parent about the tokens consumed.
-        if let Some((tokens_in, tokens_out)) = returned_usage {
-            let _ = tx.blocking_send(SubagentEvent::Usage {
-                tokens_in,
-                tokens_out,
-            });
+        let (mut tok_in, mut tok_out) = returned_usage.unwrap_or((0, 0));
+        if tok_in == 0 {
+            let prompt_chars: usize = messages.iter()
+                .filter_map(|m| m.content.as_deref())
+                .map(str::len)
+                .sum();
+            tok_in = (prompt_chars / 4).max(1) as u64;
         }
+        if tok_out == 0 {
+            let response_chars = response.content.as_deref().map_or(0, str::len);
+            tok_out = (response_chars / 4).max(1) as u64;
+        }
+        let _ = tx.blocking_send(SubagentEvent::Usage {
+            tokens_in: tok_in,
+            tokens_out: tok_out,
+        });
 
         let has_tool_calls = response.tool_calls.is_some()
             && response.tool_calls.as_ref().is_some_and(|tc| !tc.is_empty());