feat(i18n): add fallback toast and cached bundle fallback logic

1. add global i18n fallback toast on lang:fallback event, only show once per process
2. add getCachedBundleAny to retrieve expired cache for boot fallback
3. add cache fallback logic when remote i18n bundle load fails

common/js/lang.js

@@ -2,7 +2,7 @@ import { langConfig } from './config-external.js';
 import Config from './config.js';
 import http from './http.js';
 import { Platform } from './platform.js';
-import { loadBundleTree, getCachedBundle, clearBundleCache } from './lang-loader.js';
+import { loadBundleTree, getCachedBundle, getCachedBundleAny, clearBundleCache } from './lang-loader.js';
 
 // 缓存已加载的语言包
 const loadedLangPacks = {};
@@ -969,14 +969,47 @@ export default {
 				emitUpdated(lang);
 				return { ok: true, source, status: 'ok', version, lang };
 			} catch (e) {
-				// 完整性校验失败 / force 重试失败 / 任何异常
-				// 关键修复: 不再回退到 bundleCache
-				//   旧逻辑: catch → getCachedBundle → 用旧 cache(可能也是不完整 bundle)
-				//   新逻辑: catch → 不写 remoteTreeCache → $lang() 返回空 → privacy-popup 文案可能为空
-				//          但**永远不会展示"残缺文案"**, 也不会"用本地 lang 掩盖远端"
 				const _errMsg = (e && e.message) || String(e) || 'unknown';
+				// 归一化 reason:
+				//   - 'timeout'    : fetchBundle 因超时抛出
+				//   - 'http'       : 远端返回 code != 0
+				//   - 'parse'      : 签名校验失败等
+				//   - 'exception'  : 其他 JS 异常
+				const _reason = _errMsg === 'timeout' ? 'timeout'
+					: _errMsg === 'bundle sign invalid' ? 'parse'
+					: 'exception';
 				console.warn('[lang.boot] remote-only bundle load failed:', _errMsg);
-				setRemoteStatus('fail', { reason: 'exception', lang });
+
+				// 强制重试失败 / 远端拉取异常时,用任意缓存兜底(不检查 TTL)
+				//   - loadBundleTree 内部 force=true 时直接抛错(不回退),由这里兜底
+				//   - 比"bootCompleted=true + remoteTreeCache 为空 → $lang() 全返回空串"体验更好
+				//   - 至少能展示过期文案,UI 不至于全空
+				//   - 仅当内存+storage 都没缓存时,才走真正的失败路径
+				const cached = getCachedBundleAny({ lang, appType, uniacid });
+				if (cached && cached.tree) {
+					console.warn('[lang.boot] falling back to cached bundle:', cached.source);
+					remoteTreeCache[_preloadMk] = {
+						version: cached.version || '',
+						tree: cached.tree,
+						ts: Date.now(),
+						source: 'cache-fallback',
+					};
+					setRemoteStatus('fail', { reason: _reason, lang });
+					bootCompleted = true;
+					emitUpdated(lang);
+					return {
+						ok: true,
+						source: 'cache',
+						status: 'fail',
+						reason: _errMsg || 'fallback to cache',
+						lang,
+						cacheAge: cached.ts ? Date.now() - cached.ts : 0,
+					};
+				}
+
+				// 没有任何缓存(首次安装 + 远端挂) → 真正的失败
+				// $lang() 在 bootCompleted=true + remoteTreeCache 为空时返回空串(UI 降级显示空文案)
+				setRemoteStatus('fail', { reason: _reason, lang });
 				bootCompleted = true;
 				emitUpdated(lang);
 				return { ok: false, source: 'remote-failed', status: 'fail', reason: _errMsg || 'bundle failed', lang };