FN-5997: fix mobile chat empty state spacing

Remove the extra mobile right margin from chat empty states while preserving other chat pane states.

- add mobile message-pane padding and remove card chrome from empty-state containers inside mobile chat messages
- expand the mobile chat rendering regression test to cover direct threads, room threads, loading, populated, streaming, and new-chat empty states
- rename the regression suite to FN-5997 and assert the empty state spans the full mobile message pane

Files changed:
 packages/dashboard/app/components/ChatView.css     |  9 +++++
 .../__tests__/ChatView.mobile-render.test.tsx      | 46 +++++++++++++++-------
 2 files changed, 40 insertions(+), 15 deletions(-)

Fusion-Task-Id: FN-5997

Fusion-Task-Lineage: ea424660-e8c6-4691-9234-463866aacb2e

packages/dashboard/app/components/ChatView.css

@@ -1775,10 +1775,19 @@
   .chat-messages {
     min-height: 0;
     overflow-y: auto;
+    padding: var(--space-lg) var(--space-sm);
     -webkit-overflow-scrolling: touch;
     touch-action: pan-y;
   }
 
+  .chat-messages > .chat-empty-state {
+    padding: var(--space-xl) var(--space-sm);
+    border: none;
+    border-radius: 0;
+    background: transparent;
+    box-sizing: border-box;
+  }
+
   .chat-thread-header-identity {
     flex: 1 1 auto;
     min-width: 0;