feat(mobile): add scroll-to-end button

[Kapish14]

Closes #5434.

Mobile threads do not provide a quick way to return to the latest activity after scrolling through a long conversation.

This adds a floating scroll-to-end button to the shared mobile thread view. It uses the feed's existing end-follow state, calls the existing keyboard-aware scroll path, and disappears once the latest activity is visible again. Touch-down activation keeps the control responsive while the feed is moving under scroll momentum.

Testing

• Tested using a standalone preview APK on Android
• Verified that the button appears after scrolling away from the end
• Verified that tapping it during scroll momentum returns to the latest activity
• Verified that it disappears after reaching the end
• Targeted mobile lint and TypeScript checks pass
• iPhone and iPad behavior has not been tested on-device

Video

WhatsApp.Video.2026-08-10.at.05.08.19.mp4

Note

Add scroll-to-end button to mobile thread detail screen

• Adds a floating ControlPill button above the composer in ThreadDetailScreen.tsx that appears when the feed is not following the end of the message list.
• Pressing the pill triggers haptic feedback and scrolls to the end of the thread via scrollMessageToEnd.
• ThreadFeed.tsx gains an onEndFollowEnabledChange callback prop to notify the parent when end-follow state changes.
• ControlPill.tsx gains activateOnPressIn and className props; activateOnPressIn fires onPress on press-in and suppresses the subsequent release event to avoid double-invocation.

^{Macroscope summarized 205c7fd.}

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Note

Low Risk
Localized mobile UI and scroll behavior reusing existing keyboard-aware scroll APIs; no auth, data, or API changes.

Overview
Adds a floating scroll-to-end control on mobile when the user has scrolled away from the latest messages. Visibility tracks the feed’s existing end-follow latch: ThreadFeed notifies the parent via onEndFollowEnabledChange, and ThreadDetailScreen shows the button when follow is off and content is ready.

Tapping runs the existing scrollMessageToEnd path (with haptics) so the list returns to the live edge and follow re-arms when the viewport is back at the end. Thread switches reset follow to enabled.

ControlPill gains optional activateOnPressIn (fires on touch-down and skips duplicate onPress on release) and className for styling the card-like down-arrow pill above the composer.

User docs in docs/user/threads.md describe the control briefly.

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[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 2 potential issues.

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR adds a new user-facing scroll-to-end button feature on mobile. New features introducing new UI behavior and interactions warrant human review. Additionally, two unresolved Cursor findings identify potential bugs with button flashing on thread switch and scroll-to-end not re-arming live follow.

No code changes detected at 205c7fd. Prior analysis still applies.

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