feat(antigravity): show Gemini 3.6 Flash usage bars in quota tracker

getAntigravityUsage filtered the fetchAvailableModels response through a
hardcoded importantModels list that only contained 3.5 Flash, silently
dropping the 3.6 Flash quota buckets so no usage bar rendered.

open-sse/services/usage/google.js

@@ -161,6 +161,9 @@ export async function getAntigravityUsage(accessToken, providerSpecificData, pro
     if (data.models) {
       // Filter only recommended/important models (must match PROVIDER_MODELS ag ids)
       const importantModels = [
+        'gemini-3.6-flash-high',
+        'gemini-3.6-flash-medium',
+        'gemini-3.6-flash-low',
         'gemini-3-flash-agent',
         'gemini-3.5-flash-low',
         'gemini-3.5-flash-extra-low',