Support web search (#1370)

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

## Summary by cubic
Adds web search to Dyad Pro chats with a new UI, tag parsing, and a Pro
Mode toggle that wires through to the engine.

- **New Features**
  - Pro Mode toggle: “Web Search” (settings.enableProWebSearch).
  - New custom tags: dyad-web-search, dyad-web-search-result, dyad-read.
- Collapsible Web Search Result UI with in-progress badge and markdown
rendering.
- Engine integration: passes enable_web_search and activates DyadEngine
when web search is on.

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

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;
+    enableWebSearch?: boolean;
     smartContextMode?: "balanced" | "conservative";
   };
   settings: UserSettings;
@@ -158,6 +159,7 @@ export function createDyadEngine(
               enable_smart_files_context:
                 options.dyadOptions.enableSmartFilesContext,
               smart_context_mode: options.dyadOptions.smartContextMode,
+              enable_web_search: options.dyadOptions.enableWebSearch,
             };
           }