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. -->

e2e-tests/snapshots/thinking_budget.spec.ts_thinking-budget-2.txt

@@ -421,7 +421,8 @@
         }
       ],
       "enable_lazy_edits": true,
-      "enable_smart_files_context": true
+      "enable_smart_files_context": true,
+      "smart_context_mode": "balanced"
     }
   },
   "headers": {