fix: prevent permanent loading screen when navigating between pages

Two bugs in page content loading caused pages to show "Loading..." forever:

1. When readPageContent returned null (file doesn't exist on disk yet),
   pageContents[id] was never set, so contentLoaded stayed false forever.
   Fix: fall back to empty string when backend returns null.

2. The check `!pageContents[id]` treated empty string '' as falsy, causing
   re-reads from backend on every navigation to pages with empty content.
   Fix: use `!(id in pageContents)` for proper existence check.

Applied the fix in all three code paths: handlePageSelect, initial load,
and loadAndApplySpaceState (space switching).

https://claude.ai/code/session_01WsFd9hPNmHcP9kJCj7fZHM

Author: claude

packages/ui/src/components/App.tsx

@@ -168,9 +168,9 @@ export function App() {
       // Load selected page content
       if (state.selectedPageId) {
         const content = await readSpacePageContent(backend, spaceId, state.selectedPageId);
-        if (content !== null) {
-          setPageContents((prev) => ({ ...prev, [state.selectedPageId!]: content }));
-        }
+        // Set content even when null (file doesn't exist yet) — use empty string
+        // to avoid getting stuck in a permanent loading state
+        setPageContents((prev) => ({ ...prev, [state.selectedPageId!]: content ?? '' }));
       }
     } else {
       // Empty space
@@ -212,9 +212,9 @@ export function App() {
       // Load selected page content from backend
       if (persisted.selectedPageId) {
         void readPageContent(backend, persisted.selectedPageId).then((content) => {
-          if (content !== null) {
-            setPageContents((prev) => ({ ...prev, [persisted.selectedPageId!]: content }));
-          }
+          // Set content even when null (file doesn't exist yet) — use empty string
+          // to avoid getting stuck in a permanent loading state
+          setPageContents((prev) => ({ ...prev, [persisted.selectedPageId!]: content ?? '' }));
         });
       }
     } else if (shouldShowDemo) {
@@ -358,11 +358,12 @@ export function App() {
       addToRecent(id, node.title, node.icon);
     }
     // Load page content from backend if not already cached
-    if (!pageContents[id]) {
+    // Use `in` check instead of falsy check — empty string '' is valid content
+    if (!(id in pageContents)) {
       void currentReadPage(id).then((content) => {
-        if (content !== null) {
-          setPageContents((prev) => ({ ...prev, [id]: content }));
-        }
+        // Set content even when null (page file doesn't exist yet) — use empty string
+        // to avoid getting stuck in a permanent loading state
+        setPageContents((prev) => ({ ...prev, [id]: content ?? '' }));
       });
     }
     // Close sidebar on narrow screens after selecting a page