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. -->

src/ipc/utils/get_model_client.ts

@@ -84,6 +84,7 @@ export async function getModelClient(
                   ? false
                   : settings.enableProLazyEditsMode,
               enableSmartFilesContext: settings.enableProSmartFilesContextMode,
+              smartContextMode: settings.proSmartContextOption,
             },
             settings,
           })

src/ipc/utils/llm_engine_provider.ts

@@ -44,6 +44,7 @@ or to provide a custom fetch implementation for e.g. testing.
   dyadOptions: {
     enableLazyEdits?: boolean;
     enableSmartFilesContext?: boolean;
+    smartContextMode?: "balanced";
   };
   settings: UserSettings;
 }
@@ -149,6 +150,7 @@ export function createDyadEngine(
               enable_lazy_edits: options.dyadOptions.enableLazyEdits,
               enable_smart_files_context:
                 options.dyadOptions.enableSmartFilesContext,
+              smart_context_mode: options.dyadOptions.smartContextMode,
             };
           }