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-zh/strings.xml

@@ -979,6 +979,8 @@
     <string name="timeline_marker_personality_switch">人格已切换</string>
     <string name="timeline_marker_auto_continue">回合已自动继续</string>
     <string name="timeline_marker_delegation_complete">委派任务已完成</string>
+    <string name="timeline_marker_skill_invocation">技能已调用</string>
+    <string name="timeline_marker_internal_notification">系统通知</string>
     <string name="message_your_response">你的回复</string>
     <string name="moa_add_preset">添加预设</string>
     <string name="moa_add_reference">添加参考模型</string>