fix(ui): prevent visual flashing when switching tasks in detail view

[bborn]

Summary

• Fix janky/jarring visual flash when switching tasks using up/down arrows in task detail view
• Add a 500ms grace period after task transitions to prevent focus state changes from causing style flashing
• Skip focus checking during active transitions (while taskTransitionInProgress is true)

Problem

When switching tasks in the detail view, users experienced a jarring flash of text and style changes. This was caused by:

1. The focus tick (every 300ms) checking tmux focus state
2. During transitions, focus state could flip between focused/unfocused
3. Each focus change invalidated the content cache and re-rendered with different styles (bright vs dimmed borders and text)

Solution

• Added taskTransitionGraceUntil field to track when focus checks can resume
• Skip RefreshFocusState() during active transitions and for 500ms after task loads
• This gives the new detail view time to settle before focus is checked again

Test plan

• Build passes (go build ./...)
• All tests pass (go test ./...)
• Manual testing: Switch between tasks using up/down arrows in detail view - should be smooth without flashing

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