新增: 压缩失败降级前端透出与vite看门修复

- E2:LLM压缩失败走关键词摘要兜底时新发AiContextDegraded事件,
  前端toast「上下文已降级」不再静默失忆(实证76730ce8 seq166
  用户被迫原句重发);不发AiError防误判生成结束
- keyword_fallback_commit返回inserted供emit判定,双调用点
  (LLM失败/provider重建失败)透出
- vite watch忽略target/:cargo编译写.o触发EBUSY杀掉dev进程

验收:cargo check 0 error;keyword_fallback 3 passed;vue-tsc EXIT 0

src/i18n/en/aiChat.ts

@@ -275,6 +275,7 @@ export default {
     // B4: one-shot auto-compress notice (loop auto-compress no longer fully silent)
     autoCompressed: 'Context auto-compressed (earlier messages folded into summary)',
     compressError: 'Compression failed: {msg}',
+    contextDegraded: 'Context degraded: {reason} — the AI may be missing some earlier details',
     clearError: 'Clear failed: {msg}',
     initError: 'AI panel init failed: {msg} (reopen panel to retry)',
     initDegraded: 'List load failed, continuing in degraded mode: {msg}',