fix(stores): 配置文件形状不符先备份再回退；日志尾部同步落盘

- commands/connections/settings 三个 store 原先只对 JSON.parse 抛错做
  .bak 备份，合法 JSON 但形状不符（如 [1,2,3]）会静默回退空/默认，下次
  写盘把命令库/书签凭据/自定义主题整体销毁。现统一为形状不符也先备份
  （与 knownHosts 的 fail-closed 策略对齐），三个 store 测试补对应用例
- sanitizeRules 回退分支返回预设副本而非模块级共享数组引用，并在回退时
  记录 warning（原先连警告都没有）
- 会话日志尾部：logStop 触发的最后一轮 flush 改同步落盘（在途异步写未
  落地时由 drain 循环做保序的最终同步轮），before-quit 不再丢日志尾巴

tests/settings-store.mjs

@@ -94,6 +94,27 @@ console.log('[highlight rules]')
   const out = (await roundTrip({ highlightRules: [] })).highlightRules
   ok(out.length === 0, 'an explicitly empty rule list stays empty')
 }
+{
+  // A non-array highlightRules is a shape error, not "no rules": the built-in
+  // rules come back as copies (mutating a loaded list must not poison the
+  // module-level preset table) and the fallback is recorded in the warnings log.
+  writeFileSync(
+    join(userData, 'settings.json'),
+    JSON.stringify({ highlightRules: { nope: true } }),
+    'utf8'
+  )
+  const first = store.loadSettings().highlightRules
+  ok(first.length > 0, `a non-array highlightRules loads the built-in rules (got ${first.length})`)
+  const second = store.loadSettings().highlightRules
+  ok(first !== second, 'each load hands out its own array, not the shared preset list')
+  first.length = 0
+  const afterMutation = store.loadSettings().highlightRules
+  ok(afterMutation.length > 0, 'emptying a loaded rule list does not affect the next load')
+  const log = existsSync(join(userData, 'settings-warnings.log'))
+    ? readFileSync(join(userData, 'settings-warnings.log'), 'utf8')
+    : ''
+  ok(log.includes('highlightRules: not an array'), 'the fallback is recorded in settings-warnings.log')
+}
 
 // ---- 3. value bands ---------------------------------------------------------
 console.log('[value bands]')