Real-time study queue updates

[bootuz]

Summary

The study-queue surfaces computed card "dueness" as fsrsDue <= now() inside a combine { } block, which only re-runs when an upstream Flow emits. A card becoming due is pure wall-clock time crossing a threshold — no data changes at that instant — so the counts went stale until some unrelated event (an edit, a sync, or re-subscription) forced a recompute. This PR makes time itself a reactive input so the queue refreshes the exact moment a card becomes due, with no data change, navigation, or restart.

• New operator core/domain/fsrs/DueTicker.kt → Flow<List<Card>>.withDueTicks(now): pairs each card list with a now timestamp re-emitted precisely at the soonest future fsrsDue (event-driven via flatMapLatest + delay). Reschedules on card changes; idles (no busy-wait) when nothing is future-due; flatMapLatest cancels the pending wait when WhileSubscribed unsubscribes (zero background wakeups).
• StudyQueueViewModel (Today/Queue home) and DeckDetailViewModel (deck Due/New stats + the Study ↔ "You're all caught up!" slot) now feed their cards flow through the operator and consume the reactive nowMillis.

Out of scope: the active study session stays a fixed snapshot; studiedToday midnight rollover; OS background/notification refresh.

Test plan

• DueTickerTest (6 JVM tests, Turbine + virtual time): immediate emit + idle on no-future-due; re-emit exactly when a future Review card becomes due; soonest-then-next ordering; Learning-state watched; deleted card ignored; restart on card-list change.
• StudyQueueViewModelTest + LibraryAndDeckDetailViewModelTest: a near-future card starts uncounted, then appears in dueCount/readyCount after virtual time advances past its due moment.
• Full unit-test suite green; :app:assembleDebug BUILD SUCCESSFUL.
• Manual smoke (pending emulator): leave the Today/deck screen open with a card due shortly — count rises and the action slot flips at the due moment, no navigation; backgrounding past the due moment and returning shows correct counts immediately.

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