fix(desktop): keep thread-open affordance in archived channels (#4012)

## Problem

Threaded replies "disappeared" from archived Buzz channels: the **"N
replies →"** summary row and the huddle-started **"View thread"** button
vanished, so existing threads were unreachable from the channel
timeline. The thread data was intact — this was a UI gate, not data
loss.

## Root cause

A single `onReply` prop drove two distinct affordances:
- the **compose** affordances (hover "Reply" button, inline reply
target), and
- the **view** affordances ("N replies →" summary row, huddle "View
thread").

`ChannelPane` nulls `onReply` on archived channels to keep them
read-only. That correctly hid composing — but also hid the view
affordances, since they keyed off the same prop.

## Fix

Two independent props, one per concern:

- **`onReply`** drives the compose affordances and is gated on
`archivedAt` — nulled on archived channels, so no new replies can be
started.
- **`onOpenThread`** drives the view affordances and is passed
regardless of archived state, threaded `ChannelPane → MessageTimeline →
TimelineMessageList → MessageRow`.

Opening a thread on an archived channel is read-only: the thread panel's
composer is independently gated via `isComposerDisabled` (includes
`archivedAt !== null`, `ChannelPane.tsx:318`).

### Before
<img width="811" height="794" alt="Screenshot 2026-07-31 at 20 26 00"
src="https://github.com/user-attachments/assets/670d9db4-30da-4c6d-97dc-275b5dbebca8"
/>

### After
<img width="873" height="791" alt="Screenshot 2026-07-31 at 20 28 04"
src="https://github.com/user-attachments/assets/88525231-2539-4eb3-8117-8e58a0cb3855"
/>

## Validation

- `pnpm typecheck` clean
- biome lint clean on touched files
- full `pnpm test` suite green (3885 tests)
- pre-push `branch-skew` / `desktop-check` / `desktop-test` hooks passed

Signed-off-by: Trey Wood <treyw@squareup.com>
Co-authored-by: npub14h0tw3uj7jm77qfxcwn6um2s5h55l0klrt2w9srzp3m3yvjc0mpsjsuk6e <addeb74792f4b7ef0126c3a7ae6d50a5e94fbedf1ad4e2c0620c771232587ec3@buzz.block.builderlab.xyz>

desktop/src/features/channels/ui/ChannelPane.tsx

@@ -681,6 +681,7 @@ export const ChannelPane = React.memo(function ChannelPane({
             onMarkUnread={onMarkUnread}
             onMarkRead={onMarkRead}
             onReply={activeChannel?.archivedAt ? undefined : onOpenThread}
+            onOpenThread={onOpenThread}
             channelName={activeChannel?.name}
             channelType={activeChannel?.channelType ?? null}
             isSendingVideoReviewComment={isSending}

desktop/src/features/messages/ui/MessageRow.tsx

@@ -88,6 +88,7 @@ export const MessageRow = React.memo(
     onMarkRead,
     onToggleReaction,
     onReply,
+    onOpenThread,
     onEntranceComplete,
     playEntrance = false,
     onUnfollowThread,
@@ -135,6 +136,7 @@ export const MessageRow = React.memo(
       remove: boolean,
     ) => Promise<void>;
     onReply?: (message: TimelineMessage) => void;
+    onOpenThread?: (message: TimelineMessage) => void;
     onUnfollowThread?: (message: TimelineMessage) => void;
     onEntranceComplete?: (messageId: string) => void;
     playEntrance?: boolean;
@@ -335,7 +337,7 @@ export const MessageRow = React.memo(
             <HuddleAttachment
               channelId={channelId}
               message={message}
-              onOpenThread={onReply}
+              onOpenThread={onOpenThread}
             />
           );
         default:

desktop/src/features/messages/ui/MessageThreadPanel.tsx

@@ -755,6 +755,7 @@ export function MessageThreadPanel({
                         onMarkUnread={onMarkUnread}
                         onMarkRead={onMarkRead}
                         onReply={onSelectReplyTarget}
+                        onOpenThread={onExpandReplies}
                         onToggleReaction={onToggleReaction}
                         profiles={profiles}
                         showDepthGuides={shouldShowThreadBranchGuides}

desktop/src/features/messages/ui/MessageTimeline.tsx

@@ -84,6 +84,7 @@ type MessageTimelineProps = {
   onMarkUnread?: (message: TimelineMessage) => void;
   onMarkRead?: (message: TimelineMessage) => void;
   onReply?: (message: TimelineMessage) => void;
+  onOpenThread?: (message: TimelineMessage) => void;
   isSendingVideoReviewComment?: boolean;
   onSendVideoReviewComment?: (
     message: TimelineMessage,
@@ -178,6 +179,7 @@ const MessageTimelineBase = React.forwardRef<
     onMarkUnread,
     onMarkRead,
     onReply,
+    onOpenThread,
     channelName,
     channelType,
     isSendingVideoReviewComment = false,
@@ -635,6 +637,7 @@ const MessageTimelineBase = React.forwardRef<
       onMarkUnread={onMarkUnread}
       onMarkRead={onMarkRead}
       onReply={onReply}
+      onOpenThread={onOpenThread}
       isSendingVideoReviewComment={isSendingVideoReviewComment}
       onSendVideoReviewComment={onSendVideoReviewComment}
       onStartReached={loadOlderViaVirtualizer}

desktop/src/features/messages/ui/TimelineMessageList.tsx

@@ -77,6 +77,7 @@ type TimelineMessageListProps = {
   onMarkUnread?: (message: TimelineMessage) => void;
   onMarkRead?: (message: TimelineMessage) => void;
   onReply?: (message: TimelineMessage) => void;
+  onOpenThread?: (message: TimelineMessage) => void;
   isSendingVideoReviewComment?: boolean;
   onSendVideoReviewComment?: (
     message: TimelineMessage,
@@ -142,6 +143,7 @@ export const TimelineMessageList = React.memo(function TimelineMessageList({
   onMarkUnread,
   onMarkRead,
   onReply,
+  onOpenThread,
   isSendingVideoReviewComment = false,
   onSendVideoReviewComment,
   onToggleReaction,
@@ -255,6 +257,7 @@ export const TimelineMessageList = React.memo(function TimelineMessageList({
               onMarkRead={onMarkRead}
               onMarkUnread={onMarkUnread}
               onReply={onReply}
+              onOpenThread={onOpenThread}
               onToggleReaction={onToggleReaction}
               profiles={profiles}
               searchActiveMessageId={searchActiveMessageId}
@@ -286,6 +289,7 @@ export const TimelineMessageList = React.memo(function TimelineMessageList({
       onMarkRead,
       onMarkUnread,
       onReply,
+      onOpenThread,
       onToggleReaction,
       profiles,
       ownerProfiles,
@@ -699,6 +703,7 @@ type MessageRowItemProps = Pick<
   | "onMarkUnread"
   | "onMarkRead"
   | "onReply"
+  | "onOpenThread"
   | "onToggleReaction"
   | "profiles"
   | "searchActiveMessageId"
@@ -737,6 +742,7 @@ function MessageRowItem({
   onMarkUnread,
   onMarkRead,
   onReply,
+  onOpenThread,
   onToggleReaction,
   profiles,
   searchActiveMessageId,
@@ -755,7 +761,7 @@ function MessageRowItem({
   const canDelete = canManage && onDelete ? onDelete : undefined;
   const canEdit = canManage && onEdit ? onEdit : undefined;
 
-  if (summary && onReply) {
+  if (summary && onOpenThread) {
     const isHighlighted = message.id === highlightedMessageId;
     return (
       <div
@@ -790,6 +796,7 @@ function MessageRowItem({
           onMarkUnread={onMarkUnread}
           onToggleReaction={onToggleReaction}
           onReply={onReply}
+          onOpenThread={onOpenThread}
           onUnfollowThread={
             unfollowThreadById
               ? () => unfollowThreadById(message.id)
@@ -802,7 +809,7 @@ function MessageRowItem({
         <MessageThreadSummaryRow
           depth={message.depth}
           message={message}
-          onOpenThread={onReply}
+          onOpenThread={onOpenThread}
           showDepthGuides={false}
           summary={summary}
           summaryIndentOffsetRem={-THREAD_REPLY_ROW_MARGIN_INLINE_REM}
@@ -839,6 +846,7 @@ function MessageRowItem({
         onMarkUnread={onMarkUnread}
         onToggleReaction={onToggleReaction}
         onReply={onReply}
+        onOpenThread={onOpenThread}
         profiles={profiles}
         searchQuery={isSearchMatch ? searchQuery : undefined}
         showDepthGuides={false}