smart context v3 (#1022)

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

## Summary by cubic
Adds Smart Context v3 with selectable modes (Off, Conservative,
Balanced) and surfaces token savings in chat. Also improves token
estimation by counting per-file tokens when Smart Context is enabled.

- **New Features**
- Smart Context selector in Pro settings with three options.
Conservative is the default when enabled without an explicit choice.
- New setting: proSmartContextOption ("balanced"); undefined implies
Conservative.
- Engine now receives enable_smart_files_context and smart_context_mode.
- Chat shows a DyadTokenSavings card when the message contains
token-savings?original-tokens=...&smart-context-tokens=..., with percent
saved and a tooltip for exact tokens.
- Token estimation uses extracted file contents for accuracy when Pro +
Smart Context is on; otherwise falls back to formatted codebase output.

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

e2e-tests/gateway.spec.ts

@@ -7,7 +7,7 @@ testSkipIfWindows("claude 4 sonnet", async ({ po }) => {
     location: "home-chat-input-container",
   });
   await proModesDialog.toggleTurboEdits();
-  await proModesDialog.toggleSmartContext();
+  await proModesDialog.setSmartContextMode("off");
   await proModesDialog.close();
 
   await po.selectModel({ provider: "Anthropic", model: "Claude 4 Sonnet" });