fix(desktop): QA 驱动的 6 项缺陷修复

P0-C1: SecureStorage 解密失败上限 — 添加 per-key 失败计数器，
超过 2 次自动清除过期加密数据，阻断无限重试循环

P0-C2: Bootstrap 空指针防护 — connectionStore 中 relayModels[0]?.id
添加 null guard，抛出用户友好错误

P1-H1: 侧边栏对话列表去重 — ConversationList 添加按 ID 去重逻辑，
保留最新版本后按 updatedAt 排序

P1-H2: 搜索框过滤生效 — Sidebar 传递 searchQuery 给 ConversationList，
支持按标题和消息内容过滤

P1-H3: 模型选择器 fallback — 当 SaaS 和 config 均无模型时，
提供 6 个默认模型（GLM/GPT/DeepSeek/Qwen/Claude）

P1-H4: 详情面板错误友好化 — RightPanel 中 JS 错误替换为
'连接状态获取失败，请重新连接'

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

desktop/src/lib/secure-storage.ts

@@ -216,8 +216,18 @@ async function writeEncryptedLocalStorage(key: string, value: string): Promise<v
  * Read and decrypt data from localStorage
  * Supports v2 (random salt), v1 (static salt), and legacy unencrypted formats
  */
+// Track per-key decryption failures to break infinite retry loops
+const decryptionFailures = new Map<string, number>();
+const MAX_DECRYPTION_RETRIES = 2;
+
 async function readEncryptedLocalStorage(key: string): Promise<string | null> {
   try {
+    // If this key has already failed too many times, skip immediately
+    const failCount = decryptionFailures.get(key) ?? 0;
+    if (failCount >= MAX_DECRYPTION_RETRIES) {
+      return null;
+    }
+
     const encryptedKey = ENCRYPTED_PREFIX + key;
     const encryptedRaw = localStorage.getItem(encryptedKey);
 
@@ -230,12 +240,15 @@ async function readEncryptedLocalStorage(key: string): Promise<string | null> {
           const parsed = JSON.parse(encryptedRaw);
           const salt = base64ToArray(parsed.salt);
           const cryptoKey = await deriveKey(masterKeyRaw, salt);
-          return await decrypt(
+          const result = await decrypt(
             { iv: parsed.iv, data: parsed.data },
             cryptoKey,
           );
+          decryptionFailures.delete(key);
+          return result;
         } catch (error) {
-          console.error('[SecureStorage] v2 decryption failed:', error);
+          console.warn('[SecureStorage] v2 decryption failed for key:', key);
+          decryptionFailures.set(key, failCount + 1);
           // Fall through to try v1
         }
       }
@@ -245,9 +258,17 @@ async function readEncryptedLocalStorage(key: string): Promise<string | null> {
         try {
           const cryptoKey = await deriveKey(masterKeyRaw); // uses legacy static salt
           const encrypted = JSON.parse(encryptedRaw);
-          return await decrypt(encrypted, cryptoKey);
+          const result = await decrypt(encrypted, cryptoKey);
+          decryptionFailures.delete(key);
+          return result;
         } catch (error) {
-          console.error('[SecureStorage] v1 decryption failed:', error);
+          console.warn('[SecureStorage] v1 decryption failed for key:', key);
+          decryptionFailures.set(key, failCount + 1);
+          // Both v2 and v1 failed — clear stale encrypted data to stop retrying
+          if (failCount + 1 >= MAX_DECRYPTION_RETRIES) {
+            localStorage.removeItem(encryptedKey);
+            console.warn('[SecureStorage] Cleared stale encrypted data for key:', key);
+          }
         }
       }
     }