feat(zookeeper): add UTF-8 value preview

Closes #6897

apps/desktop/src/lib/__tests__/kv/kvValueDisplay.spec.ts

@@ -1,8 +1,31 @@
 import { describe, expect, it } from "vitest";
-import { formatZooKeeperMetadataRows, formatZooKeeperSummaryBadges, prettyPrintJsonText } from "@/lib/kv/kvValueDisplay";
+import { decodeBase64Utf8Preview, formatZooKeeperMetadataRows, formatZooKeeperSummaryBadges, prettyPrintJsonText } from "@/lib/kv/kvValueDisplay";
 import type { KvKeyMetadata } from "@/lib/backend/api";
 
 describe("kv value display helpers", () => {
+  it("decodes a valid Base64 value as a lossless UTF-8 preview", () => {
+    expect(decodeBase64Utf8Preview("5L2g5aW9")).toEqual({
+      ok: true,
+      value: "你好",
+      lossy: false,
+    });
+  });
+
+  it("marks replacement decoding of invalid UTF-8 bytes as lossy", () => {
+    expect(decodeBase64Utf8Preview("ZoBv")).toEqual({
+      ok: true,
+      value: "f�o",
+      lossy: true,
+    });
+  });
+
+  it("rejects malformed Base64 without throwing", () => {
+    expect(decodeBase64Utf8Preview("not base64!")).toEqual({
+      ok: false,
+      error: "invalid_base64",
+    });
+  });
+
   it("pretty prints valid JSON text", () => {
     expect(prettyPrintJsonText('{"name":"张三","roles":["admin","reader"]}')).toEqual({
       ok: true,

apps/desktop/src/lib/kv/kvValueDisplay.ts

@@ -8,6 +8,8 @@ export interface PrettyPrintJsonResult {
   error?: "invalid_json";
 }
 
+export type Base64Utf8PreviewResult = { ok: true; value: string; lossy: boolean } | { ok: false; error: "invalid_base64" };
+
 export interface DisplayItem {
   label: string;
   value: string;
@@ -20,6 +22,33 @@ export interface ZooKeeperSummaryLabels {
   size?: string;
 }
 
+const strictBase64Pattern = /^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$/;
+
+export function decodeBase64Utf8Preview(value: string): Base64Utf8PreviewResult {
+  const normalized = value.replace(/\s+/g, "");
+  if (!strictBase64Pattern.test(normalized)) return { ok: false, error: "invalid_base64" };
+
+  try {
+    const binary = atob(normalized);
+    const bytes = Uint8Array.from(binary, (character) => character.charCodeAt(0));
+    try {
+      return {
+        ok: true,
+        value: new TextDecoder("utf-8", { fatal: true }).decode(bytes),
+        lossy: false,
+      };
+    } catch {
+      return {
+        ok: true,
+        value: new TextDecoder("utf-8").decode(bytes),
+        lossy: true,
+      };
+    }
+  } catch {
+    return { ok: false, error: "invalid_base64" };
+  }
+}
+
 export function prettyPrintJsonText(text: string): PrettyPrintJsonResult {
   try {
     return { ok: true, value: safeJsonFormat(text, 2) };