feat(frontend): display token usage per conversation turn (#1229)

Surface the usage_metadata that PR #1218 added to the streaming API.
A compact indicator in the chat header shows cumulative tokens consumed
per thread, with a tooltip breakdown of input/output/total counts.

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

frontend/src/core/i18n/locales/en-US.ts

@@ -274,6 +274,12 @@ export const enUS: Translations = {
     failed: "Subtask failed",
   },
 
+  // Token Usage
+  tokenUsage: {
+    title: "Token Usage",
+    input: "Input",
+    output: "Output",
+    total: "Total",
   // Shortcuts
   shortcuts: {
     searchActions: "Search actions...",