fix(ui): add gemini-3-flash-preview to model dropdowns

Added gemini-3-flash-preview model to agy and gemini provider catalogs
in the UI. This enables users to select the fast Gemini 3 Flash model
from dropdown menus.

Closes #194

ui/src/lib/model-catalogs.ts

@@ -53,7 +53,18 @@ export const MODEL_CATALOGS: Record<string, ProviderCatalog> = {
           default: 'gemini-3-pro-preview',
           opus: 'gemini-3-pro-preview',
           sonnet: 'gemini-3-pro-preview',
-          haiku: 'gemini-2.5-flash',
+          haiku: 'gemini-3-flash-preview',
+        },
+      },
+      {
+        id: 'gemini-3-flash-preview',
+        name: 'Gemini 3 Flash',
+        description: 'Fast Gemini model via Antigravity',
+        presetMapping: {
+          default: 'gemini-3-flash-preview',
+          opus: 'gemini-3-pro-preview',
+          sonnet: 'gemini-3-pro-preview',
+          haiku: 'gemini-3-flash-preview',
         },
       },
     ],
@@ -72,7 +83,19 @@ export const MODEL_CATALOGS: Record<string, ProviderCatalog> = {
           default: 'gemini-3-pro-preview',
           opus: 'gemini-3-pro-preview',
           sonnet: 'gemini-3-pro-preview',
-          haiku: 'gemini-2.5-flash',
+          haiku: 'gemini-3-flash-preview',
+        },
+      },
+      {
+        id: 'gemini-3-flash-preview',
+        name: 'Gemini 3 Flash',
+        tier: 'paid',
+        description: 'Fast Gemini 3 model, requires paid Google account',
+        presetMapping: {
+          default: 'gemini-3-flash-preview',
+          opus: 'gemini-3-pro-preview',
+          sonnet: 'gemini-3-pro-preview',
+          haiku: 'gemini-3-flash-preview',
         },
       },
       {