Add subtitle sidebar startup auto-open and resume jump

- Add `subtitleSidebar.autoOpen` with startup-only open behavior
- Jump to the first resolved active cue on initial resume position
- Clear parsed cues when subtitle prefetch init fails

changes/2026-03-21-subtitle-sidebar-auto-open.md

@@ -0,0 +1,4 @@
+type: fixed
+area: overlay
+
+- Added `subtitleSidebar.autoOpen` (default `false`) to open the subtitle sidebar once during overlay startup when the sidebar feature is enabled.

changes/2026-03-21-subtitle-sidebar-resume-jump.md

@@ -0,0 +1,4 @@
+type: fixed
+area: overlay
+
+- Made subtitle sidebar resume/start positioning jump directly to the first resolved active cue instead of smooth-scrolling through the full list, while keeping smooth auto-follow for later cue changes.