Add document viewer with proper 404 handling for trust center

Move document download/view to a dedicated viewer page with PDF preview,
access request flow, and a proper 404 error boundary when documents are
not found. The backend now returns NOT_FOUND instead of INTERNAL for
missing documents and reports.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/server/api/trust/v1/schema.graphql

@@ -874,7 +874,7 @@ type RecordSigningEventPayload {
 
 type Query {
   viewer: Identity
-  node(id: ID!): Node!
+  node(id: ID!): Node
   currentTrustCenter: TrustCenter
 }