Mobile: threads open at the top instead of the newest reply (desktop opens at bottom)

### Describe the bug

Opening a thread with many replies on the mobile app lands at the **top** of the thread — the user has to scroll all the way down to reach the newest reply. The desktop thread panel opens scrolled to the **bottom** (newest message), so the two clients disagree on where a thread opens.

### To reproduce

1. On mobile, open a channel and tap into a thread whose replies overflow the screen (say 20+ replies).
2. The view shows the thread head at the top; the newest replies are below the fold.
3. Same thread on desktop: the panel opens pinned to the newest message.

### Expected behavior

Mobile matches desktop: opening a thread lands on the newest reply (with short threads still showing the head at the top).

### Mechanism (as of `28ae6cd`)

This looks like a deliberate-layout change that completed only half of desktop's behavior — said with respect, since the current behavior is clearly chosen, not accidental: #3485 converted the thread list from `reverse: true` to top-anchored (fixing short threads jamming the head against the composer) and pinned the head-visible hydration with a named test. Its in-code comment says the layout matches "desktop's thread panel" — and it does match desktop's DOM order, but desktop pairs that top-down layout with an explicit bottom-pin on open: `useAnchoredScroll`'s `pinToBottomOnMount()` (`desktop/src/features/messages/ui/useAnchoredScroll.ts`) scrolls to `scrollHeight` when the thread panel initializes, with a deep-link target taking precedence and a missed target falling back to the bottom-pin.

Mobile's `thread_detail_page.dart` performs no initial scroll unless `initialMessageId` is set — the hydration effect explicitly establishes its baseline "without moving the user away from the head". The earlier fix for this exact symptom (#1499, reversed list) was superseded by #3485's redesign.

So the parity claim is incomplete rather than wrong: top-down layout ✔, open-at-latest ✘.

### Fix

PR incoming from this account: keep #3485's top-anchored layout (short threads keep the head at top), add a one-shot initial pin to the newest reply mirroring desktop's precedence (resolvable deep-link target wins; missed target falls back to the pin; a user who scrolled during a slow hydration is never overridden), widget tests included — the head-visible hydration test is flipped to the completed contract.