feat(web): enhanced file panel — side-by-side diff, hide whitespace, inline review

Adds DiffSplitView component for side-by-side diff mode, whitespace-only
change filtering, inline review comments with thread/gutter cell UI, diff
preferences persistence, and write-file API support for in-browser editing.

Backend: hideWhitespace param on git diff endpoint, write_file route.

apps/web/src/components/InlineReviewGutterCell.tsx

@@ -0,0 +1,43 @@
+import { cn } from '@/lib/utils';
+import { Plus } from 'lucide-react';
+
+interface InlineReviewGutterCellProps {
+  lineNumber: number | null;
+  type: 'add' | 'remove' | 'context' | 'header' | null;
+  hasComments: boolean;
+  canComment: boolean;
+  onClick?: () => void;
+}
+
+export function InlineReviewGutterCell({
+  lineNumber,
+  type,
+  hasComments,
+  canComment,
+  onClick,
+}: InlineReviewGutterCellProps) {
+  return (
+    <div
+      className={cn(
+        'relative flex items-center justify-end pr-1 min-w-[40px] h-5 text-[11px] font-mono select-none',
+        type === 'add' && 'bg-green-950/30',
+        type === 'remove' && 'bg-red-950/30',
+        type === 'context' && 'bg-muted/10',
+        canComment && 'cursor-pointer group',
+      )}
+      onClick={canComment ? onClick : undefined}
+    >
+      <span className="text-muted-foreground/70">
+        {lineNumber != null ? lineNumber : ''}
+      </span>
+      {canComment && (
+        <span className="absolute left-0.5 hidden group-hover:flex items-center justify-center w-4 h-4 rounded text-muted-foreground hover:text-foreground">
+          <Plus size={12} />
+        </span>
+      )}
+      {hasComments && (
+        <span className="absolute left-0.5 w-1.5 h-1.5 rounded-full bg-blue-400" />
+      )}
+    </div>
+  );
+}