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/engine.spec.ts

@@ -10,13 +10,13 @@ testSkipIfWindows("send message to engine", async ({ po }) => {
 });
 
 testSkipIfWindows(
-  "send message to engine - smart context balanced",
+  "send message to engine - smart context conservative",
   async ({ po }) => {
     await po.setUpDyadPro();
     const proModesDialog = await po.openProModesDialog({
       location: "home-chat-input-container",
     });
-    await proModesDialog.setSmartContextMode("balanced");
+    await proModesDialog.setSmartContextMode("conservative");
     await proModesDialog.close();
     await po.selectModel({ provider: "Google", model: "Gemini 2.5 Pro" });
     await po.sendPrompt("[dump] tc=turbo-edits");