修复: i18n 裸 @ 致生产环境对话输入框消失

- 905 将 @ 改为裸 @,而 vue-i18n 的 @ 是 linked-message 语法字符,未转义只在 prod runtime 抛 Invalid linked format(dev 宽松回退/vue-tsc 捕获不到)
- zh/en aiChat 8 处(placeholderDefault/mentionHint/searchFilesBlocked/projectLabel)按规范转义为 {'@'}
- 输入框 placeholder 消息编译抛错→ChatInput 渲染失败→输入框消失,即本次生产回归根因
- 验证: 全库 462 个含 @/{/| 的 i18n 值经 message-compiler 编译 0 失败 + vue-tsc EXIT 0

src/i18n/en/aiChat.ts

@@ -137,12 +137,12 @@ export default {
     clearSkill: 'Clear skill',
     skillBadgeTip: 'Injected full skill playbook: {name}',
     placeholderWithSkill: 'Skill selected. Type arguments and press Enter',
-    placeholderDefault: "Type a message... (/ for skills, @ to mention)",
+    placeholderDefault: "Type a message... (/ for skills, {'@'} to mention)",
     skillNoMatch: 'No matching skill',
     skillNotLoaded: 'No local skills loaded',
 
     // ── @ entity mention (@ opens popover, select inserts [Type:Name] token) ──
-    mentionHint: "@ to mention project/task",
+    mentionHint: "{'@'} to mention project/task",
     mentionGroupProject: 'Projects',
     mentionGroupTask: 'Tasks',
     mentionGroupIdea: 'Ideas',
@@ -233,7 +233,7 @@ export default {
 
     // ── Input Augmentation layer: mention/skill injection feedback ──
     // search_files blind search denied by whitelist fallback (no auth dialog; prompt user to use @mention or absolute path)
-    searchFilesBlocked: "search_files only works inside authorized dirs. Use @[project] to reference or provide an absolute path.",
+    searchFilesBlocked: "search_files only works inside authorized dirs. Use {'@'}[project] to reference or provide an absolute path.",
     // /skill hot reload (ai_reload_skills IPC) success toast
     skillReloaded: 'Skills reloaded',
     // @mention entity resolve failed (id points to a project/task/idea/skill that was deleted or unavailable)
@@ -327,7 +327,7 @@ export default {
     // ── TopBar goal/project context labels ──
     goalsCount: 'Goals ({n})',
     none: 'None',
-    projectLabel: "@Projects",
+    projectLabel: "{'@'}Projects",
 
     // ── 3c UX batch addition (I5 classified send-failure messages): friendly text by
     //    error kind; {msg} keeps the raw error string for troubleshooting ──