feat: syntax highlighting in chat input textarea

Real-time markdown syntax coloring while typing using a transparent
textarea overlay approach. Highlights: code blocks, inline code,
bold, italic, headings, and links. Toggle button (highlighter icon)
to enable/disable, persisted in localStorage.

src/components/ChatInput.tsx

@@ -1,6 +1,7 @@
 import { useState, useRef, useEffect, useCallback, lazy, Suspense } from 'react';
-import { Send, Square, Paperclip, X, FileText, Eye, EyeOff } from 'lucide-react';
+import { Send, Square, Paperclip, X, FileText, Eye, EyeOff, Highlighter } from 'lucide-react';
 import { useT } from '../hooks/useLocale';
+import { HighlightedTextarea } from './HighlightedTextarea';
 
 const ReactMarkdown = lazy(() => import('react-markdown'));
 const remarkGfm = import('remark-gfm').then(m => m.default);
@@ -91,6 +92,7 @@ export function ChatInput({ onSend, onAbort, isGenerating, disabled, sessionKey
   const [files, setFiles] = useState<FileAttachment[]>([]);
   const [isDragOver, setIsDragOver] = useState(false);
   const [showPreview, setShowPreview] = useState(() => localStorage.getItem('pinchchat-md-preview') === '1');
+  const [highlightEnabled, setHighlightEnabled] = useState(() => localStorage.getItem('pinchchat-syntax-hl') !== '0');
   const textareaRef = useRef<HTMLTextAreaElement>(null);
   const fileInputRef = useRef<HTMLInputElement>(null);
 
@@ -283,6 +285,15 @@ export function ChatInput({ onSend, onAbort, isGenerating, disabled, sessionKey
             >
               {showPreview ? <EyeOff size={18} /> : <Eye size={18} />}
             </button>
+            {/* Syntax highlight toggle */}
+            <button
+              onClick={() => setHighlightEnabled(v => { const next = !v; localStorage.setItem('pinchchat-syntax-hl', next ? '1' : '0'); return next; })}
+              className={`shrink-0 h-11 w-11 rounded-2xl border border-pc-border bg-pc-elevated/30 flex items-center justify-center transition-colors ${highlightEnabled ? 'text-pc-accent-light bg-[var(--pc-accent-glow)]' : 'text-pc-text-secondary hover:text-pc-accent-light hover:bg-[var(--pc-hover)]'}`}
+              title={highlightEnabled ? 'Disable syntax highlight' : 'Enable syntax highlight'}
+              aria-label={highlightEnabled ? 'Disable syntax highlight' : 'Enable syntax highlight'}
+            >
+              <Highlighter size={18} />
+            </button>
             <input
               ref={fileInputRef}
               type="file"
@@ -292,10 +303,11 @@ export function ChatInput({ onSend, onAbort, isGenerating, disabled, sessionKey
               accept="image/*,.pdf,.txt,.md,.json,.csv,.log,.py,.js,.ts,.tsx,.jsx,.html,.css,.yaml,.yml,.xml,.sql,.sh,.env,.toml"
             />
 
-            <textarea
+            <HighlightedTextarea
               ref={textareaRef}
               value={text}
-              onChange={(e) => setText(e.target.value)}
+              highlightEnabled={highlightEnabled}
+              onChange={(e) => setText((e.target as HTMLTextAreaElement).value)}
               onKeyDown={handleKeyDown}
               onPaste={handlePaste}
               placeholder={t('chat.inputPlaceholder')}

src/components/HighlightedTextarea.tsx

@@ -0,0 +1,97 @@
+import { forwardRef, useRef, useEffect, useImperativeHandle, useCallback } from 'react';
+
+/**
+ * A textarea with a syntax-highlighted backdrop overlay.
+ * The textarea text is transparent; a <pre> behind it renders colored tokens.
+ */
+
+interface Props extends Omit<React.TextareaHTMLAttributes<HTMLTextAreaElement>, 'style'> {
+  value: string;
+  highlightEnabled?: boolean;
+}
+
+// Simple markdown tokenizer — returns HTML with <span> wrappers
+function highlightMarkdown(text: string): string {
+  if (!text) return '\n'; // pre needs at least a newline to size correctly
+
+  // Escape HTML first
+  let html = text
+    .replace(/&/g, '&amp;')
+    .replace(/</g, '&lt;')
+    .replace(/>/g, '&gt;');
+
+  // Fenced code blocks: ```...```
+  html = html.replace(/(```[\s\S]*?```)/g, '<span class="ht-code-block">$1</span>');
+
+  // Inline code: `...`  (but not inside code blocks — already wrapped)
+  html = html.replace(/(?<!<span class="ht-code-block">[\s\S]*?)(`[^`\n]+?`)/g, '<span class="ht-inline-code">$1</span>');
+
+  // Bold: **...**
+  html = html.replace(/(\*\*[^*]+?\*\*)/g, '<span class="ht-bold">$1</span>');
+
+  // Italic: *...* (single, not inside bold)
+  html = html.replace(/(?<!\*)(\*[^*\n]+?\*)(?!\*)/g, '<span class="ht-italic">$1</span>');
+
+  // Headers at line start: # ...
+  html = html.replace(/(^|\n)(#{1,6}\s[^\n]*)/g, '$1<span class="ht-heading">$2</span>');
+
+  // Links: [text](url)
+  html = html.replace(/(\[[^\]]*\]\([^)]*\))/g, '<span class="ht-link">$1</span>');
+
+  // Ensure trailing newline for proper sizing
+  if (!html.endsWith('\n')) html += '\n';
+
+  return html;
+}
+
+export const HighlightedTextarea = forwardRef<HTMLTextAreaElement, Props>(
+  ({ value, highlightEnabled = true, className = '', ...props }, ref) => {
+    const textareaRef = useRef<HTMLTextAreaElement>(null);
+    const backdropRef = useRef<HTMLPreElement>(null);
+
+    useImperativeHandle(ref, () => textareaRef.current!);
+
+    // Sync scroll
+    const syncScroll = useCallback(() => {
+      if (textareaRef.current && backdropRef.current) {
+        backdropRef.current.scrollTop = textareaRef.current.scrollTop;
+        backdropRef.current.scrollLeft = textareaRef.current.scrollLeft;
+      }
+    }, []);
+
+    useEffect(() => {
+      syncScroll();
+    }, [value, syncScroll]);
+
+    if (!highlightEnabled) {
+      return (
+        <textarea
+          ref={textareaRef}
+          value={value}
+          className={className}
+          {...props}
+        />
+      );
+    }
+
+    return (
+      <div className="ht-container relative">
+        <pre
+          ref={backdropRef}
+          className={`ht-backdrop ${className}`}
+          aria-hidden="true"
+          dangerouslySetInnerHTML={{ __html: highlightMarkdown(value) }}
+        />
+        <textarea
+          ref={textareaRef}
+          value={value}
+          className={`ht-textarea ${className}`}
+          onScroll={syncScroll}
+          {...props}
+        />
+      </div>
+    );
+  }
+);
+
+HighlightedTextarea.displayName = 'HighlightedTextarea';

src/index.css

@@ -177,6 +177,81 @@ html, body {
 .markdown-body th { background: var(--pc-accent-glow); }
 .markdown-body img { max-width: 100%; border-radius: 8px; }
 
+/* Highlighted textarea overlay */
+.ht-container {
+  position: relative;
+  flex: 1;
+}
+
+.ht-backdrop,
+.ht-textarea {
+  /* Must share identical text layout */
+  font-family: inherit;
+  font-size: 0.875rem; /* text-sm */
+  line-height: 1.25rem;
+  padding: 0.75rem 1rem; /* py-3 px-4 */
+  white-space: pre-wrap;
+  word-break: break-word;
+  overflow-wrap: break-word;
+  overflow-x: hidden;
+  overflow-y: auto;
+  margin: 0;
+  box-sizing: border-box;
+}
+
+.ht-backdrop {
+  position: absolute;
+  inset: 0;
+  pointer-events: none;
+  color: var(--pc-text-primary);
+  border: 1px solid transparent;
+  border-radius: 1rem; /* rounded-2xl */
+  max-height: 200px;
+}
+
+.ht-textarea {
+  position: relative;
+  color: transparent;
+  caret-color: var(--pc-text-primary);
+  background: transparent !important;
+  resize: none;
+  z-index: 1;
+}
+
+/* Token colors */
+.ht-code-block {
+  color: #67e8f9; /* cyan-300 */
+  background: var(--pc-accent-glow);
+  border-radius: 4px;
+}
+
+.ht-inline-code {
+  color: #67e8f9;
+  background: var(--pc-accent-glow);
+  border-radius: 3px;
+  padding: 0 2px;
+}
+
+.ht-bold {
+  color: var(--pc-text-primary);
+  font-weight: 700;
+}
+
+.ht-italic {
+  color: var(--pc-text-secondary);
+  font-style: italic;
+}
+
+.ht-heading {
+  color: #a78bfa; /* violet-400 */
+  font-weight: 600;
+}
+
+.ht-link {
+  color: #67e8f9;
+  text-decoration: underline;
+}
+
 /* Accessibility: respect reduced-motion preferences */
 @media (prefers-reduced-motion: reduce) {
   .animate-fade-in {