feat: line numbers in code blocks with toggle button

- Show line numbers for code blocks with more than 3 lines
- Toggle button (# icon + line count) in the language header bar
- Preference persisted in localStorage
- Line numbers are non-selectable (won't copy with code)
- Hidden for short snippets to reduce visual noise

src/components/CodeBlock.tsx

@@ -1,5 +1,5 @@
 import { useState, useCallback, type HTMLAttributes, type ReactElement } from 'react';
-import { Check, Copy } from 'lucide-react';
+import { Check, Copy, Hash } from 'lucide-react';
 
 /** Extract the language from the nested <code> element's className (e.g. "language-ts"). */
 function extractLanguage(children: React.ReactNode): string | null {
@@ -42,29 +42,73 @@ function formatLanguage(lang: string): string {
  * Custom <pre> renderer for ReactMarkdown.
  * Wraps code blocks with a language label and a floating copy button.
  */
+const LINE_NUMBER_KEY = 'pinchchat-line-numbers';
+const LINE_THRESHOLD = 3; // Only show line numbers for blocks with more than this many lines
+
 export function CodeBlock(props: HTMLAttributes<HTMLPreElement>) {
   const [copied, setCopied] = useState(false);
+  const [showLineNumbers, setShowLineNumbers] = useState(() => {
+    const stored = localStorage.getItem(LINE_NUMBER_KEY);
+    return stored === null ? true : stored === 'true';
+  });
   const language = extractLanguage(props.children);
 
+  const code = (props.children as ReactElement<{ children?: string }> | undefined)?.props?.children;
+  const lines = typeof code === 'string' ? code.replace(/\n$/, '').split('\n') : [];
+  const hasEnoughLines = lines.length > LINE_THRESHOLD;
+
   const handleCopy = useCallback(() => {
-    // Extract text from the nested <code> element
-    const code = (props.children as ReactElement<{ children?: string }> | undefined)?.props?.children;
     if (typeof code === 'string') {
       navigator.clipboard.writeText(code).then(() => {
         setCopied(true);
         setTimeout(() => setCopied(false), 2000);
       });
     }
-  }, [props.children]);
+  }, [code]);
+
+  const toggleLineNumbers = useCallback(() => {
+    setShowLineNumbers(prev => {
+      const next = !prev;
+      localStorage.setItem(LINE_NUMBER_KEY, String(next));
+      return next;
+    });
+  }, []);
+
+  const shouldShowNumbers = showLineNumbers && hasEnoughLines;
 
   return (
     <div className="group/code relative">
       {language && (
         <div className="flex items-center justify-between px-4 py-1.5 bg-pc-elevated/80 border-b border-pc-border rounded-t-lg text-[11px] text-pc-text-muted font-mono select-none">
-          {formatLanguage(language)}
+          <span>{formatLanguage(language)}</span>
+          {hasEnoughLines && (
+            <button
+              onClick={toggleLineNumbers}
+              className="flex items-center gap-1 px-1.5 py-0.5 rounded hover:bg-pc-border/40 transition-colors text-pc-text-muted hover:text-pc-text-secondary"
+              title={showLineNumbers ? 'Hide line numbers' : 'Show line numbers'}
+              type="button"
+            >
+              <Hash className="h-3 w-3" />
+              <span className="text-[10px]">{lines.length}</span>
+            </button>
+          )}
         </div>
       )}
-      <pre {...props} className={`${props.className || ''} ${language ? '!rounded-t-none !mt-0' : ''}`} />
+      {shouldShowNumbers ? (
+        <div className={`flex ${language ? 'rounded-t-none' : 'rounded-lg'} overflow-hidden`}>
+          <div
+            className="flex-shrink-0 select-none text-right pr-3 pl-3 py-4 text-[11px] leading-[1.7142857] font-mono text-pc-text-muted/40 bg-pc-elevated/40 border-r border-pc-border/50"
+            aria-hidden="true"
+          >
+            {lines.map((_, i) => (
+              <div key={i}>{i + 1}</div>
+            ))}
+          </div>
+          <pre {...props} className={`${props.className || ''} flex-1 !rounded-none !mt-0 min-w-0`} />
+        </div>
+      ) : (
+        <pre {...props} className={`${props.className || ''} ${language ? '!rounded-t-none !mt-0' : ''}`} />
+      )}
       <button
         onClick={handleCopy}
         className="absolute top-2 right-2 p-1.5 rounded-lg bg-pc-elevated/60 hover:bg-pc-elevated/80 border border-pc-border-strong text-pc-text-secondary hover:text-pc-text opacity-0 group-hover/code:opacity-100 transition-opacity duration-150"