FN-5757: prevent chat message list from snapping to top

Keep chat scroll position stable when stale top snapshots are captured during mobile message updates.

- Capture scroll anchors from the first visible message instead of always sampling the first DOM node
- Ignore stale or likely-invalid zero-position snapshots before attempting scroll restoration
- Guard bottom anchoring from interrupting user scroll unless explicitly forced and add regression coverage for stale snapshot behavior

Files changed:
 packages/dashboard/app/components/ChatView.css     | 11 +++++-
 packages/dashboard/app/components/ChatView.tsx     | 33 +++++++++++++----
 packages/dashboard/app/components/__tests__/ChatView.scroll-to-top.test.tsx      | 41 ++++++++++++++++++++++
 3 files changed, 78 insertions(+), 7 deletions(-)

Fusion-Task-Id: FN-5757

Fusion-Task-Lineage: 4e4f6843-ba70-40da-8d16-358bdfcc7b09

packages/dashboard/app/components/ChatView.css

@@ -642,13 +642,15 @@
 
 /* Messages */
 .chat-messages {
-  flex: 1;
+  flex: 1 1 auto;
   min-height: 0;
   overflow-y: auto;
   padding: 16px;
   display: flex;
   flex-direction: column;
   gap: 12px;
+  -webkit-overflow-scrolling: touch;
+  touch-action: pan-y;
   /* Stop iOS rubber-band scroll from propagating to the parent /
      document when the user swipes on the edge of the messages list
      with the keyboard up. Without this, the bounce escapes the
@@ -1743,6 +1745,13 @@
     flex-wrap: nowrap;
   }
 
+  .chat-messages {
+    min-height: 0;
+    overflow-y: auto;
+    -webkit-overflow-scrolling: touch;
+    touch-action: pan-y;
+  }
+
   .chat-thread-header-identity {
     flex: 1 1 auto;
     min-width: 0;