fix: /simplify 리뷰 — UX/코드 품질 10건 수정

- EditableSegment: readOnly prop 추가, hover 배경색 효과
- MeetingDetailTabs: useMemo(parseSegments), 이중 debounce 제거, cleanup
- MeetingChatPanel: sessionId deps, 에러 표시, key=timestamp
- ExportMenu: i18n 위반 수정 (하드코딩 → t())
- MeetingModePage: 에러 Snackbar 피드백 (startRecording, viewDetail)
- TranscriptTab: polish 실패 피드백
- 12개 locale i18n 키 5개 추가

src/renderer/i18n/en.json

@@ -466,5 +466,10 @@
   "meeting.copyToClipboard": "Copy to Clipboard",
   "meeting.copied": "Copied",
   "meeting.chatCollapse": "Collapse Chat",
-  "meeting.chatExpand": "Expand Chat"
+  "meeting.chatExpand": "Expand Chat",
+  "meeting.exportMd": "Markdown (.md)",
+  "meeting.exportPdfFmt": "PDF (.pdf)",
+  "meeting.polishFailed": "AI polishing failed",
+  "meeting.viewDetailError": "Failed to load meeting details",
+  "meeting.startRecordingError": "Failed to start recording"
 }