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/lib/schemas.ts

@@ -164,7 +164,7 @@ export const UserSettingsSchema = z.object({
   thinkingBudget: z.enum(["low", "medium", "high"]).optional(),
   enableProLazyEditsMode: z.boolean().optional(),
   enableProSmartFilesContextMode: z.boolean().optional(),
-  proSmartContextOption: z.enum(["balanced"]).optional(),
+  proSmartContextOption: z.enum(["balanced", "conservative"]).optional(),
   selectedTemplateId: z.string(),
   enableSupabaseWriteSqlMigration: z.boolean().optional(),
   selectedChatMode: ChatModeSchema.optional(),