chat: peer-switch strip, back button moved to flyout header

Switching conversations used to require bouncing back to the list view
and picking from there. Now the thread view has a horizontal peer
strip at the top: one click hops to another conversation, active tab
is highlighted and auto-scrolls into view, unread peers show a dot.
Scrollbar is hidden — vertical wheel delta is redirected to horizontal
scroll so the user can slide the strip with the mouse wheel when it
overflows.

In-thread header is gone; the back button moved next to "chat" in the
flyout header and only shows while a thread is open. Peer name stays
visible via the active tab.

Bumps to 1.9.3.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/plugins/chat/chat.html

@@ -3,7 +3,9 @@
 <div id="chat-flyout" class="hidden">
     <div id="chat-flyout-header">
         <i class="ph ph-chats-circle text-t-accent text-sm"></i>
-        <span id="chat-flyout-title" class="font-pixel text-[10px] text-t-bright flex-1">chat</span>
+        <span id="chat-flyout-title" class="font-pixel text-[10px] text-t-bright">chat</span>
+        <button id="chat-header-back" class="hidden text-t-muted hover:text-t-accent transition-colors" data-tip="back to list"><i class="ph ph-caret-left text-sm"></i></button>
+        <span class="flex-1"></span>
         <button id="chat-new-btn" class="text-t-muted hover:text-t-accent transition-colors" data-tip="new conversation"><i class="ph ph-plus-square text-sm"></i></button>
         <button id="chat-refresh-btn" class="text-t-muted hover:text-t-accent transition-colors" data-tip="refresh"><i class="ph ph-arrows-clockwise text-sm" id="chat-refresh-icon"></i></button>
         <button id="chat-close-btn" class="text-t-muted hover:text-t-danger transition-colors"><i class="ph ph-x text-sm"></i></button>
@@ -21,11 +23,7 @@
 
     <!-- Conversation thread view -->
     <div id="chat-thread-view" class="chat-view hidden">
-        <div id="chat-thread-header">
-            <button id="chat-thread-back" class="text-t-muted hover:text-t-accent transition-colors"><i class="ph ph-caret-left text-sm"></i></button>
-            <i class="ph ph-user-circle text-t-accent text-sm"></i>
-            <span id="chat-thread-peer" class="font-pixel text-[10px] text-t-bright flex-1 truncate"></span>
-        </div>
+        <div id="chat-thread-peers" class="chat-thread-peers"></div>
         <div id="chat-thread-messages" class="chat-thread-messages"></div>
         <form id="chat-thread-form">
             <input id="chat-thread-input" type="text" autocomplete="off" placeholder="type a message..." maxlength="2000">