fix(chat): allow swipe-to-dismiss on the conversation sheet

[bmc08gt]

Problem

Swiping down on a chat sheet starts to move it but snaps back before reaching the dismiss threshold — the sheet can't be dismissed by drag.

Root cause

The chat message list (MessageList.kt) is a reverseLayout = true LazyColumn (newest at the bottom, index 0 = bottom), but it had .sheetResignmentBehavior(listState) applied. That modifier is built for top-anchored lists — and this was the only reversed-list usage among the 9 in the app.

The guard decides "am I at the sheet's downward dismiss boundary?" via firstVisibleItemIndex == 0 && firstVisibleItemScrollOffset == 0. For a normal list that's the top scroll edge. For the reversed chat list, index0/offset0 is the resting position (newest message), not a scroll edge:

1. At rest the guard forces the sheet's allowDismiss flag to false (it gates confirmDetentChange for the whole sheet).
2. Swiping down lets the reversed list consume the drag as a history scroll → firstVisibleItemScrollOffset changes → the modifier flips into onScrolledAway(), which disables gestures with no auto-reset and cancels the 700 ms re-enable timer.
3. allowDismiss stays false, so the sheet can't settle to Hidden and snaps back — even for header/handle drags.

Fix

Remove .sheetResignmentBehavior(listState) (and its now-unused import) from the chat list. Its model — "only allow dismiss at the list's scroll edge" — doesn't fit a chat: a downward drag on messages already scrolls history via nested scroll, and dismiss is meant to come from the header/handle drag, scrim tap, and back.

Testing

• :apps:flipcash:features:messenger:compileDebugKotlin passes.
• Manual: open a chat, swipe the sheet down — it now reaches the threshold and dismisses.