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/helpers/test_helper.ts

@@ -67,8 +67,12 @@ class ProModesDialog {
     public close: () => Promise<void>,
   ) {}
 
-  async toggleSmartContext() {
-    await this.page.getByRole("switch", { name: "Smart Context" }).click();
+  async setSmartContextMode(mode: "balanced" | "off" | "conservative") {
+    await this.page
+      .getByRole("button", {
+        name: mode.charAt(0).toUpperCase() + mode.slice(1),
+      })
+      .click();
   }
 
   async toggleTurboEdits() {