Make balanced smart context option the default (#1186)

<!-- This is an auto-generated description by cubic. -->

## Summary by cubic
Set “balanced” as the default smart context mode. Users now get balanced
when Smart Files Context is enabled and no mode is set; “conservative”
must be explicitly selected.

- **Refactors**
- Default fallback to balanced in UI and engine (proSmartContextOption
undefined -> "balanced").
- ProModeSelector saves "conservative" explicitly; selector reads
undefined as balanced.
  - Updated schema and types to allow "balanced" | "conservative".
- Engine payload now includes smart_context_mode with "balanced" by
default; e2e tests and snapshots updated.

- **Migration**
- No action needed. Existing users without an explicit mode will use
balanced by default; selecting conservative persists.

<!-- End of auto-generated description by cubic. -->

src/components/ProModeSelector.tsx

@@ -36,7 +36,7 @@ export function ProModeSelector() {
     } else if (newValue === "conservative") {
       updateSettings({
         enableProSmartFilesContextMode: true,
-        proSmartContextOption: undefined, // Conservative is the default when enabled but no option set
+        proSmartContextOption: "conservative",
       });
     } else if (newValue === "balanced") {
       updateSettings({
@@ -197,8 +197,12 @@ function SmartContextSelector({
     if (settings?.proSmartContextOption === "balanced") {
       return "balanced";
     }
-    // If enabled but no option set (undefined/falsey), it's conservative
-    return "conservative";
+    if (settings?.proSmartContextOption === "conservative") {
+      return "conservative";
+    }
+    // Keep in sync with getModelClient in get_model_client.ts
+    // If enabled but no option set (undefined/falsey), it's balanced
+    return "balanced";
   };
 
   const currentValue = getCurrentValue();