feat: auto-format JSON display everywhere

- Shared prettyJson/truncate/isJson utility in src/utils/format.ts
- MessageStream: collapsed preview + expanded view both auto-format JSON
- LogPanel: log details with JSON content auto-formatted
- PresetList: payload preview auto-formatted
- Deduplicated isJson/formatPayload logic

src/utils/format.ts

@@ -0,0 +1,33 @@
+/**
+ * Pretty-print a JSON string if it's valid JSON.
+ * Returns the original string if it's not valid JSON.
+ */
+export function prettyJson(str: string): string {
+  if (!str || str.trim() === '') return str
+  try {
+    const parsed = JSON.parse(str)
+    return JSON.stringify(parsed, null, 2)
+  } catch {
+    return str
+  }
+}
+
+/**
+ * Check if a string is valid JSON.
+ */
+export function isJson(str: string): boolean {
+  try {
+    JSON.parse(str)
+    return true
+  } catch {
+    return false
+  }
+}
+
+/**
+ * Truncate a string to a maximum length, adding an ellipsis if truncated.
+ */
+export function truncate(str: string, maxLen: number): string {
+  if (str.length <= maxLen) return str
+  return str.substring(0, maxLen) + '...'
+}