feat(ui): Implement token usage summary widget

tests/test_token_usage.py

@@ -0,0 +1,35 @@
+import pytest
+
+def test_token_usage_aggregation():
+    # A dummy test to fulfill the Red Phase for the new token usage widget.
+    # We will implement a function in gui.py or ai_client.py to aggregate tokens.
+    from ai_client import _comms_log, clear_comms_log, _append_comms
+    
+    clear_comms_log()
+    
+    _append_comms("IN", "response", {
+        "usage": {
+            "input_tokens": 100,
+            "output_tokens": 50,
+            "cache_read_input_tokens": 10,
+            "cache_creation_input_tokens": 5
+        }
+    })
+    
+    _append_comms("IN", "response", {
+        "usage": {
+            "input_tokens": 200,
+            "output_tokens": 100,
+            "cache_read_input_tokens": 20,
+            "cache_creation_input_tokens": 0
+        }
+    })
+    
+    # We expect a new function get_total_token_usage() to exist
+    from gui import get_total_token_usage
+    
+    totals = get_total_token_usage()
+    assert totals["input_tokens"] == 300
+    assert totals["output_tokens"] == 150
+    assert totals["cache_read_input_tokens"] == 30
+    assert totals["cache_creation_input_tokens"] == 5