Usage popover: label rows as session totals

Section header 'Session totals' + pluralized cache rows make the
cumulative semantics explicit.

surfaces/gui/src/components/Composer.tsx

@@ -650,7 +650,7 @@ function UsageChip({
               </div>
             ) : null}
             <div className="text-[10.5px] uppercase tracking-[0.06em] text-faint font-semibold mb-1">
-              This session
+              Session totals
             </div>
             <div className="flex flex-col gap-1.5">
               {Object.entries(usage.byModel).map(([id, t]) => (
@@ -660,8 +660,8 @@ function UsageChip({
                   </div>
                   <div className="mt-0.5 flex flex-col gap-0.5">
                     {stat("Input", t.input)}
-                    {t.cache_read > 0 && stat("Cache read", t.cache_read)}
-                    {t.cache_write > 0 && stat("Cache write", t.cache_write)}
+                    {t.cache_read > 0 && stat("Cache reads", t.cache_read)}
+                    {t.cache_write > 0 && stat("Cache writes", t.cache_write)}
                     {/* Cumulative billed input volume = fresh + cache reads + cache
                         writes (each class bills at its own rate). Shown only when a
                         cache split exists — otherwise it would duplicate Input. */}