feat: add web URL content fetcher for chat context

Add ability to fetch and inject web page content into chat as context.
Includes SSRF protection (blocks private IPs, localhost), content
extraction (strips scripts/styles/nav), and a clean popover UI.

Reimplements the concept from PR #1703 without the issues (duplicated
ChatBox, dual API routes, SSRF vulnerability, window.prompt UX).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/components/chat/Chat.client.tsx

@@ -594,6 +594,20 @@ export const ChatImpl = memo(
       Cookies.set('selectedProvider', newProvider.name, { expires: 30 });
     };
 
+    const handleWebSearchResult = useCallback(
+      (result: string) => {
+        const currentInput = input || '';
+        const newInput = currentInput.length > 0 ? `${result}\n\n${currentInput}` : result;
+
+        // Update the input via the same mechanism as handleInputChange
+        const syntheticEvent = {
+          target: { value: newInput },
+        } as React.ChangeEvent<HTMLTextAreaElement>;
+        handleInputChange(syntheticEvent);
+      },
+      [input, handleInputChange],
+    );
+
     return (
       <BaseChat
         ref={animationScope}
@@ -664,6 +678,7 @@ export const ChatImpl = memo(
         selectedElement={selectedElement}
         setSelectedElement={setSelectedElement}
         addToolResult={addToolResult}
+        onWebSearchResult={handleWebSearchResult}
       />
     );
   },