style: add print stylesheet for conversation export

- Hide sidebar, header, chat input, and scroll button when printing
- Reset backgrounds to white, text to black for readability
- Code blocks get light background with borders
- Tool calls styled for print (compact, bordered)
- External links show their URL in parentheses
- Chat scroll area becomes fully visible (no overflow)
- Messages avoid page-break-inside for clean pagination
- Remove shadows and backdrop filters in print

src/components/ChatInput.tsx

@@ -222,7 +222,7 @@ export function ChatInput({ onSend, onAbort, isGenerating, disabled, sessionKey
 
   return (
     <div
-      className="border-t border-pc-border bg-[var(--pc-bg-input)]/60 backdrop-blur-xl p-4"
+      className="border-t border-pc-border bg-[var(--pc-bg-input)]/60 backdrop-blur-xl p-4 print-hide"
       role="form"
       aria-label={t('chat.inputLabel')}
       onDragOver={handleDragOver}