PanelHandle snapshots DockPanel::title at registration and the tab strip
renders that cache, so switching the UI language at runtime (menu or
preferences) left every docked tab in the previous language. Capture a
type-erased title reader and notifier from the concrete panel entity and
add DockArea::refresh_panel_titles, which re-reads each title, marks the
panel view dirty for localized content, and repaints the chrome.
Also fix the timeline_view test fixture that missed ClipRenderData's
multicam field (gpui's test target did not compile).
