relay: surface shared-bucket activity in compose sidebar

Relay UI already shows a tenant's inbox + sent mails. Add a parallel
view for collaborative bucket cards the logged-in tenant is a member
of, so activity on shared boards is visible without needing the hyprcrd
client open.

New `GET /api/collab-inbox` endpoint (session-auth only) decrypts
payloads with the inter-tenant key and returns title + preview + author
+ bucket metadata. UI restructures the compose view into editor + right
sidebar stacking inbox → shared → sent, and polls all three on the
existing 15s tick.

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

relay/ui/layout.php

@@ -105,16 +105,22 @@ if (!defined('RELAY_BOOTSTRAPPED')) { http_response_code(404); exit; }
         </div>
         <div id="compose-status" class="compose-status-bar"></div>
     </div>
-    <div id="inbox-section" class="section">
-        <h2><i class="ph ph-tray"></i> your inbox <span class="count" id="inbox-count">0</span>
-            <span style="flex:1"></span>
-            <button id="inbox-delete-all" class="btn btn-danger hidden" style="padding:3px 10px;font-size:9px;" onclick="deleteAllMessages()"><i class="ph ph-trash"></i> delete all</button>
-        </h2>
-        <div id="inbox-list"></div>
-    </div>
-    <div id="outbox-section" class="section">
-        <h2><i class="ph ph-paper-plane-tilt"></i> sent <span class="count" id="outbox-count">0</span></h2>
-        <div id="outbox-list"></div>
+    <div id="compose-sidebar" class="compose-sidebar">
+        <div id="inbox-section" class="section">
+            <h2><i class="ph ph-tray"></i> your inbox <span class="count" id="inbox-count">0</span>
+                <span style="flex:1"></span>
+                <button id="inbox-delete-all" class="btn btn-danger hidden" style="padding:3px 10px;font-size:9px;" onclick="deleteAllMessages()"><i class="ph ph-trash"></i> delete all</button>
+            </h2>
+            <div id="inbox-list"></div>
+        </div>
+        <div id="shared-section" class="section">
+            <h2><i class="ph ph-users-three"></i> shared buckets <span class="count" id="shared-count">0</span></h2>
+            <div id="shared-list"></div>
+        </div>
+        <div id="outbox-section" class="section">
+            <h2><i class="ph ph-paper-plane-tilt"></i> sent <span class="count" id="outbox-count">0</span></h2>
+            <div id="outbox-list"></div>
+        </div>
     </div>
 </div>