fix(i18n): fall back to built-in zh table when a pack misses a key

A stale .pjson (e.g. downloaded before new keys existed) showed raw 'msg.xxx'
because pmm_tr looked only in the loaded pack. Now a missing key falls back to
the built-in zh-CN table, so any pack stays readable.

Also sync the mirror language packs (zh-CN/en-US) with the new keys
(desc.clean, msg.dep-satisfied, msg.err.conflict, msg.cache-cleared,
msg.self-up-to-date); users just re-run 'pmm setting lang <loc>' to pick it up.

src/i18n.c

@@ -8,6 +8,7 @@
 #include <stdarg.h>
 
 static JsonValue *g_lang = NULL;
+static JsonValue *g_builtin = NULL;
 static char g_locale[64] = "";
 
 /* Detect locale from LANG / LC_ALL env, else default "zh-CN". */
@@ -80,9 +81,18 @@ int pmm_lang_load_active(void) {
 }
 
 const char *pmm_tr(const char *key) {
-    if (!g_lang) return key;
+    /* Look up in the loaded pack first, then fall back to the built-in zh-CN
-    const char *t = json_str(g_lang, key);
+     * table so a stale .pjson never shows a raw "msg.xxx" key. */
-    return t ? t : key;
+    if (g_lang) {
+        const char *t = json_str(g_lang, key);
+        if (t) return t;
+    }
+    if (!g_builtin) g_builtin = json_parse(kBuiltinZh);
+    if (g_builtin) {
+        const char *b = json_str(g_builtin, key);
+        if (b) return b;
+    }
+    return key;
 }
 
 const char *pmm_tr_fmt(const char *key, ...) {

web/mirror/lang/en-US.pjson

@@ -123,5 +123,10 @@
   "mirror.sources": "mirrors:",
   "mirror.checking": "checking registry mirrors...",
   "mirror.result": "result: %d/%d sources reachable",
-  "lang.packs-in": "language packs in %s:"
+  "lang.packs-in": "language packs in %s:",
+  "desc.clean": "clean cache",
+  "msg.dep-satisfied": "dependency %s%s%s already installed/provided, skipped",
+  "msg.err.conflict": "cannot install %s: conflicts with installed %s",
+  "msg.cache-cleared": "cache cleaned: %d files, freed %.1f KB",
+  "msg.self-up-to-date": "already up to date (v%s, latest v%s), skipping upgrade"
 }

web/mirror/lang/zh-CN.pjson

@@ -123,5 +123,10 @@
   "mirror.sources": "镜像源:",
   "mirror.checking": "正在检查注册表镜像源...",
   "mirror.result": "结果: %d/%d 个镜像源可达",
-  "lang.packs-in": "%s 中的语言包:"
+  "lang.packs-in": "%s 中的语言包:",
+  "desc.clean": "清理缓存",
+  "msg.dep-satisfied": "依赖 %s%s%s 已装/被提供,跳过",
+  "msg.err.conflict": "无法安装 %s:与已装的 %s 冲突",
+  "msg.cache-cleared": "已清理缓存: %d 个文件，释放 %.1f KB",
+  "msg.self-up-to-date": "当前 v%s 已是最新（v%s），跳过升级"
 }