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/ipc/utils/get_model_client.ts

@@ -85,7 +85,8 @@ export async function getModelClient(
                   ? false
                   : settings.enableProLazyEditsMode,
               enableSmartFilesContext: settings.enableProSmartFilesContextMode,
-              smartContextMode: settings.proSmartContextOption,
+              // Keep in sync with getCurrentValue in ProModeSelector.tsx
+              smartContextMode: settings.proSmartContextOption ?? "balanced",
             },
             settings,
           })