task 004: replace time.Sleep goroutine synchronization with proper primitives

audio_player.go: use time.AfterFunc instead of goroutine+sleep for the
100ms auto-play delay — no goroutine is blocked waiting.

app.go (setupUI, setupTooltips): replace goroutine+sleep with time.AfterFunc
for both 500ms tooltip initialization delays.

navigation.go: replace fixed 500ms sleep in the delete-cleanup goroutine with
a ticker+select loop that polls hasActiveOperations(). The goroutine now
proceeds as soon as all in-flight operations for the deleted word finish
(or after a 5-second safety timeout), rather than waiting a fixed interval
that may be too short or too long.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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