feat(web): export the session log from the trajectory toolbar

The 导出 button fetches GET /api/session.export and saves the ZIP (root
artifact plus subagent descendants). The plugin exposes exportLog through
the view's inject face, resolves the tab label through the locale service,
and disables the button while in flight; fixture mode answers 404 so the
error bar explains the gap.

apps/web/tests/snapshots/navigation-panes/trajectory.expected.md

@@ -2,6 +2,7 @@
   - button "Use actual duration": Duration
   - button "Collapse turns": Turns
   - button "Collapse calls": Calls
+  - button "导出会话日志": 导出
   - img
   - searchbox "Search trajectory"
 - region "Trajectory timeline":