Auto-expanding textareas and proper article comment box

Add useAutoResize hook that grows textareas to fit content up to a max.
Applied to compose box, inline replies, thread replies, and article
comments. Article comment input changed from single-line input to a
multi-row textarea with Shift+Enter support.

src/hooks/useAutoResize.ts

@@ -0,0 +1,24 @@
+import { useCallback } from "react";
+
+/**
+ * Auto-resize a textarea to fit its content, up to maxRows.
+ * Returns an onChange handler that should be spread onto the textarea.
+ * Usage: <textarea onChange={autoResize} ... />
+ */
+export function useAutoResize(minRows = 2, maxRows = 10) {
+  const autoResize = useCallback(
+    (e: React.ChangeEvent<HTMLTextAreaElement>) => {
+      const ta = e.target;
+      // Reset to min height to measure scrollHeight accurately
+      ta.style.height = "auto";
+      const lineHeight = parseInt(getComputedStyle(ta).lineHeight) || 18;
+      const minHeight = lineHeight * minRows;
+      const maxHeight = lineHeight * maxRows;
+      const newHeight = Math.min(Math.max(ta.scrollHeight, minHeight), maxHeight);
+      ta.style.height = `${newHeight}px`;
+    },
+    [minRows, maxRows],
+  );
+
+  return autoResize;
+}