feat(mobile): add channel scroll navigation

[tellaho]

Category: improvement
User Impact: Mobile readers can jump directly to their oldest unread message and return to the latest message with compact directional controls.
Problem: Opening an active channel at its newest message makes it easy to miss where unread conversation began, while moving back through history lacks a lightweight route to the live edge.
Solution: Capture the channel's unread boundary when it opens, offer an accessible up-chevron beneath the app bar to reach that stable target, then reveal the inverse down-chevron at the bottom whenever the reader is away from latest. Deep links retain precedence, and live-follow, pagination, composer resizing, and explicit scroll ownership continue to use the existing timeline behavior.

File changes

mobile/lib/features/channels/channel_detail_page.dart
Captures the channel's read state at open time and passes a stable unread snapshot into the timeline before the normal deferred read update advances it.

mobile/lib/features/channels/channel_detail_page/message_list.dart
Adds mutually exclusive oldest-unread and latest navigation, with accessible icon controls positioned at opposite edges of the message surface while preserving existing follow and deep-link behavior.

mobile/test/features/channels/channel_detail_page_test.dart
Covers the unread target, compact inverse controls, accessible tooltips, and placement beneath the frosted app bar.

Reproduction steps

1. Open a Flutter mobile channel that has unread messages without entering through a message or thread deep link.
2. Confirm an up-chevron appears directly below the channel app bar while the timeline remains at latest.
3. Tap the up-chevron and confirm the timeline scrolls to the oldest message that was unread when the channel opened.
4. Confirm the unread control is replaced by a down-chevron at the bottom of the timeline.
5. Tap the down-chevron and confirm the timeline returns to latest and resumes following new messages.

Screenshots

At latest — up-chevron to oldest unread	Away from latest — down-chevron to latest

Real iPhone 17 Pro Simulator captures from the neutral buzz-mobile-scroll-to channel.

Originating Buzz thread: buzz://message?channel=5b16c478-22d8-4ddd-951a-6036e19b81ff&id=6a78af32d7ac6f531b182c4e70dd5a04c503a2dab2ce2c0c74b2c6baa5921741&thread=6a78af32d7ac6f531b182c4e70dd5a04c503a2dab2ce2c0c74b2c6baa5921741

[tellaho]

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