fix: remove red current time indicator line from sidebar timeline

[devin-ai-integration]

Summary

Removes the red horizontal line (CurrentTimeIndicator) from the sidebar timeline, as raised in #3830. The line was confusing to users who didn't understand its purpose.

The CurrentTimeIndicator component is kept as an invisible zero-height anchor div so the "Go back to now" scroll functionality continues to work. The red-tinted inset shadow on the "Go back to now" button is also replaced with a neutral shadow-sm.

Review & Testing Checklist for Human

• Verify scroll-to-now still works: The invisible anchor div replaces the visible red line — confirm the "Go back to now" button still correctly scrolls to the current time position in the timeline
• Check timeline item spacing: The old indicator had px-3 py-2 padding that created vertical gaps between timeline items. With the padding removed, items near the current time boundary may appear closer together — verify this looks acceptable
• Visual check of the "Go back to now" button: Confirm the button still looks fine with shadow-sm instead of the previous red-tinted inset shadow

Notes

• Requested by @ComputelessComputer
• Devin session

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[devin-ai-integration]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.