refactor: move save status indicator to sticky overlay in editor area

[j4rviscmd]

Summary

• Move SaveStatusIndicator from the auto-hide header to a sticky overlay inside the scroll container so it remains visible regardless of header visibility
• Replace null return with an invisible placeholder to prevent layout shifts when the indicator appears/disappears
• Fix stale closure in useEffect by using isTimerActiveRef instead of reading display state

Details

SaveStatusIndicator changes

• Unified the two wrapper <span> elements into a single return with conditional invisible class via cn()
• Replaced the display state read inside useEffect with an isTimerActiveRef ref to avoid stale closures and satisfy react-hooks/exhaustive-deps
• Fixed clearTimeout non-null assertion by using undefined instead of null for the ref type

App.tsx changes

• Removed SaveStatusIndicator from the <header> element
• Added a sticky top-5 overlay inside the scroll container with pointer-events-none so it doesn't interfere with scrolling
• Flattened the header ternary to cn() conditional class pattern

Test plan

• Scroll down to hide the header — save indicator should remain visible
• Trigger auto-save — checkmark should appear without layout shift
• Verify no scroll event interference from the overlay (pointer-events-none)
• Confirm error state ("Save failed") displays correctly

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