perf: avoid O(N^2) indexOf in notifications list

[barrydeen]

Summary

• Users (notably on GrapheneOS) reported the app freezing when tapping the Notifications bottom-nav tab.
• Root cause: inside the LazyColumn items { … } lambda we called notifications.indexOf(item) on every item composition. That's a linear scan per item → O(N²) total work per recomposition pass, with each scan invoking FlatNotificationItem.equals (data class). With a few hundred notifications and frequent profileVersion bumps invalidating the list, this is enough to stall the main thread long enough to look like a freeze on slower hardware.
• Fix: switch to itemsIndexed(...) and use the supplied index directly. One-line behavioral equivalent, no more quadratic scan.

Test plan

• Open the Notifications tab on a populated account; verify list renders and scrolls smoothly
• Tap a notification to expand/collapse (uses itemIndex for animateScrollToItem) and confirm scroll-to-item still lands correctly when replying
• Pull to refresh; confirm ordering and summary header are unchanged
• Ideally: have a GrapheneOS user confirm the freeze on tab switch is gone