fix: prevent save status indicator from persisting on note switch

[j4rviscmd]

Summary

• Remove isTimerActiveRef guard that caused a deadlock when save status transitioned from 'saved' to 'idle' during note navigation
• The timer was cleared via clearTimeout but the ref was never reset, leaving the checkmark icon visible indefinitely
• Add cleanup function to clear timer on component unmount

Root Cause

In SaveStatusIndicator.tsx, when status changed from 'saved' → 'idle' (note switch):

1. clearTimeout() killed the 3-second timer
2. isTimerActiveRef.current remained true (never reset since timer callback never fires)
3. setDisplay(null) was guarded by !isTimerActiveRef.current, creating a deadlock
4. The checkmark stayed visible forever

Fix

• Remove isTimerActiveRef entirely
• Always set display to null when status === 'idle'
• Add useEffect cleanup function to clear timer on unmount

Test plan

• Edit a note and wait for the auto-save checkmark to appear
• Switch to another note while the checkmark is visible
• Verify the checkmark disappears immediately on note switch
• Verify the checkmark still shows normally when editing and saving

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