chat: group chat UI — room tabs, creation flow, member mgmt

Generalises the flyout from peer-only to a thread abstraction
({kind: 'peer'|'room', key, label}). Conversations list and peer
strip render rooms with a group icon alongside 1:1 peers; clicking
either opens the same thread view.

New UI surface:
  - Pick view gets a 1:1 / group mode switch. Group mode reveals a
    name field + multi-select member list and a Create button.
  - Room threads show an info icon in the flyout header that opens a
    member list; owners can add/remove members, rename-later, or
    delete the room; non-owners can leave.
  - 1:1 threads show a disposable toggle (ph-fire) in the header;
    flipping it tells the relay to drop future sends after delivery
    instead of persisting.
  - Group bubbles show the author's username above each message when
    it changes, matching common chat UX.

Thread open triggers a backfill fetch so cold caches / second
devices catch up on older history from the relay.

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

public/plugins/chat/chat.html

@@ -6,6 +6,8 @@
         <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-thread-disposable" class="hidden text-t-muted hover:text-t-accent transition-colors" data-tip="disposable: messages drop after delivery"><i class="ph ph-fire text-sm"></i></button>
+        <button id="chat-thread-info" class="hidden text-t-muted hover:text-t-accent transition-colors" data-tip="room info"><i class="ph ph-info text-sm"></i></button>
         <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>
@@ -31,12 +33,35 @@
         </form>
     </div>
 
-    <!-- New-conversation peer picker -->
+    <!-- New-conversation peer picker / group creation.
+         Shared view: a mode toggle at top switches between picking one
+         peer (1:1) and building a group (name + multi-select). -->
     <div id="chat-pick-view" class="chat-view hidden">
         <div id="chat-pick-header">
             <button id="chat-pick-back" class="text-t-muted hover:text-t-accent transition-colors"><i class="ph ph-caret-left text-sm"></i></button>
-            <span class="font-pixel text-[10px] text-t-bright flex-1">pick a recipient</span>
+            <span id="chat-pick-title" class="font-pixel text-[10px] text-t-bright flex-1">new conversation</span>
         </div>
+        <div id="chat-pick-mode" class="chat-pick-mode">
+            <button class="chat-pick-mode-btn active" data-mode="peer" type="button">1:1</button>
+            <button class="chat-pick-mode-btn" data-mode="group" type="button">group</button>
+        </div>
+        <input id="chat-group-name" class="chat-group-name hidden" type="text" autocomplete="off" placeholder="group name..." maxlength="80">
         <div id="chat-pick-list" class="chat-pick-list"></div>
+        <div id="chat-group-actions" class="chat-group-actions hidden">
+            <span id="chat-group-count" class="font-pixel text-[9px] text-t-dim"></span>
+            <span style="flex:1"></span>
+            <button id="chat-group-create" type="button" class="px-2 py-1 text-[10px] font-pixel border border-t-accent text-t-accent hover:bg-t-accent/10 transition-colors">Create</button>
+        </div>
+    </div>
+
+    <!-- Room info / member management view.
+         Shown when the user taps the info icon in a room thread. -->
+    <div id="chat-room-info-view" class="chat-view hidden">
+        <div id="chat-room-info-header">
+            <button id="chat-room-info-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-users-three text-t-accent text-sm"></i>
+            <span id="chat-room-info-name" class="font-pixel text-[10px] text-t-bright flex-1 truncate"></span>
+        </div>
+        <div id="chat-room-info-body" class="chat-room-info-body"></div>
     </div>
 </div>