feat: Enhance tab UX with drag & drop, middle-click close, context menu, and session management, and resolve tab flicker.

docs/development/devlog/2025-12-06-011500_tab_flicker_fix.md

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+# Tab Flicker Fix
+
+## Issue
+The user reports a flicker when clicking a tab before it becomes active.
+
+## Diagnosis
+The current implementation of the active tab style uses `borderBottom: isActive ? 2 : 0`. This causes a layout shift (height change or content displacement) of 2px whenever the active state changes. This visual jump is perceived as a flicker.
+
+## Solution
+Update the styling to maintain a constant border width but change the color.
+-   Change `borderBottom` to always be `2`.
+-   Change `borderColor` to be `'primary.main'` when active and `'transparent'` when inactive.
+
+## Plan
+1.  Modify `src/frontend/src/components/TabsBar.tsx`.
+2.  Update `SortableTab` styles.