fix(chat): give every backend-tagged system event its own timeline chip (#936)

System events ride as role=user rows with a display_kind sidecar (backend
issues #904/#82888). The full-bleed renderer routes any displayKind != null
to a SystemEvent, but timelineMarkerLabelRes only had labels for
model_switch / personality_switch / auto_continue / async_delegation_complete.
skill_invocation and internal_notification had NO label, so TimelineMarkerChip
returned early and they fell through to the bare SystemBubble (centered italic
text, no clear system identity) instead of a distinct chip.

- Add skill_invocation + internal_notification to timelineMarkerLabelRes.
- Add string resources en/zh/ko (ko was missing all timeline markers; added
  the two new ones for parity).
- TimelineMarkerTest: assert both new kinds map to labels.

Result: every backend system message type now renders as its own centered
timeline chip, never as a user bubble. Legacy sessions persisted before the
backend added display_kind tagging still lack the sidecar and would need a
one-time backend re-tag to stop rendering as user bubbles.

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/res/values-ko/strings.xml

@@ -752,4 +752,6 @@
     <string name="image_viewer_save_failed">이미지를 저장할 수 없음</string>
     <string name="image_viewer_load_failed">이미지를 불러올 수 없음: %1$s</string>
     <string name="image_viewer_share_failed">이미지를 공유할 수 없음</string>
+    <string name="timeline_marker_skill_invocation">스킬 호출됨</string>
+    <string name="timeline_marker_internal_notification">시스템 알림</string>
 </resources>