feat: add red blur indicator for active tab position in header

[devin-ai-integration]

Summary

Adds a red blur indicator to the header tab bar that shows when the currently active tab is scrolled out of view. The indicator appears on the left side when the active tab is scrolled off to the left, and on the right side when scrolled off to the right.

Changes:

• Added useActiveTabPosition hook that tracks whether the active tab is visible, scrolled left, or scrolled right
• Modified useScrollActiveTabIntoView to expose tabRefsMap for position calculations
• Added red blur indicator elements that render conditionally based on active tab position

Review & Testing Checklist for Human

• Verify visual appearance: Open the app with multiple tabs, scroll the tab bar, and confirm the red blur indicator appears on the correct side when the active tab is out of view
• Check blur visibility: The blur uses w-3 h-3 blur-sm bg-red-500 - verify this is visible enough against the header background
• Test edge cases: Check behavior when active tab is partially visible, when there's only one tab, and when switching between tabs while scrolled

Test Plan

1. Open the desktop app
2. Create enough tabs to require scrolling in the header
3. Select a tab, then scroll the tab bar so the selected tab is out of view
4. Verify a red blur appears on the side where the active tab is located
5. Scroll back to make the active tab visible and verify the blur disappears

Notes

• Link to Devin run: https://app.devin.ai/sessions/427db388e56449fd8ba285aedb412c4b
• Requested by: @ComputelessComputer

[devin-ai-integration]

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