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/style.css

@@ -687,13 +687,19 @@ header .nav-btn .ph { font-size:11px; vertical-align:-1px; margin-right:2px; }
         border-radius: var(--radius);
     }
 
-    /* Compose view: 2-column grid */
+    /* Compose view: 2 columns. Left is the compose editor. Right is a
+       sidebar with inbox → shared → sent stacked vertically. */
     #view-compose:not(.hidden) {
         display: grid !important;
         grid-template-columns: minmax(0, 1.7fr) minmax(0, 1fr);
         gap: 24px;
         align-items: start;
     }
+    #compose-sidebar {
+        display: flex;
+        flex-direction: column;
+        gap: 16px;
+    }
 
     /* Cards: depth via shadow + larger padding */
     .compose-card,