fix(web): reset sourceEverLoadedRef on file/project switch

`sourceEverLoadedRef` was marked as permanently-true once set, meaning
it never reset when the user navigated to a different file or project.
The render guard at the loading skeleton checks
`source === null && !sourceEverLoadedRef.current`; with the ref stuck
at `true`, a file/project switch skipped the loading skeleton entirely
while `source` was still null, leaving a blank iframe visible during
a slow fetch.

Fix: reset `sourceEverLoadedRef.current = false` in the existing
file/project-switch reset effect (deps: `[projectId, file.name]`).
The fetch effect sets it back to `true` only after the current file's
source arrives successfully — exactly as before.

Codex P2 finding. Refs #4650

apps/web/src/components/FileViewer.tsx

@@ -5371,6 +5371,12 @@ function HtmlViewer({
     setCommentPanelOpen(false);
     setCommentCreateMode(false);
     setActivePreviewCommentId(null);
+    // Reset the "ever loaded" sentinel so the loading skeleton is shown again
+    // while the new file's source is being fetched. Without this reset the
+    // sentinel stays true from the previous file, the render guard skips the
+    // skeleton, and a slow fetch leaves the user staring at a blank iframe
+    // instead of the loading indicator (codex P2 finding, issue #4650).
+    sourceEverLoadedRef.current = false;
   }, [projectId, file.name]);
   const activePreviewSrcUrl = (
     previewSrcUrl === effectiveBasePreviewSrcUrl ||