feat(collections): show loading spinner during stale-content reloads

Document reader and collection document list only showed a spinner on
first load — refetching over already-visible content (reopening a
document, remote-changes refresh) had no loading indicator at all.

Outpost/Features/Collections/CollectionOverviewView.swift

@@ -65,6 +65,16 @@ struct CollectionOverviewView: View {
                 }
             }
             .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .topLeading)
+            // Corner spinner for reloads with stale data already on screen
+            // (remote-changes refresh, tab switch after the first load) —
+            // `documentList`'s own spinner only covers the empty-state case.
+            .overlay(alignment: .topTrailing) {
+                if viewModel.isLoading && !viewModel.documents.isEmpty {
+                    ProgressView()
+                        .controlSize(.small)
+                        .padding(12)
+                }
+            }
         }
         // No `.navigationTitle` here — it renders its own native title bubble,
         // duplicating the leading toolbar item `ContentView_macOS` already

Outpost/Features/Collections/DocumentReaderView.swift

@@ -49,6 +49,17 @@ struct DocumentReaderView: View {
             .padding()
         }
         // No `.navigationTitle` here either — same reason as CollectionOverviewView.
+        // Corner spinner, not gated on `text.isEmpty`: `viewModel.text` starts
+        // pre-filled from the list/summary copy of the doc, so the full-page
+        // spinner above rarely fires on open — without this, the re-fetch in
+        // `loadFullContent()` looked like nothing was happening.
+        .overlay(alignment: .topTrailing) {
+            if viewModel.isLoading && !viewModel.text.isEmpty {
+                ProgressView()
+                    .controlSize(.small)
+                    .padding(12)
+            }
+        }
         .task { await viewModel.loadFullContent() }
     }