[AlexAndBear]

Description

• Folder loading errors: apps that load their files themselves (currently the preview) get them via useAppFolderHandling, which swallowed the load error. A deep link to a file that doesn't exist therefore kept spinning forever in the preview. The error is now kept as folderLoadingError and the app wrapper shows it like its own loading errors, so every app using the folder loading benefits.

Note: rebased onto #3590, so a 404 here shows the "Resource not found" message with the ghost that #3590 added to the app wrapper.

Before (after 25 s)	After

Related Issue

• No issue, found in the functional bug sweep of the web client

How Has This Been Tested?

• test environment: local dev server, Playwright (Chromium)
• /preview/personal/admin/<folder>/nope.png and a path in a missing folder: error screen "Resource not found", no spinner, no pageerror
• an existing image still opens in the preview
• unit tests added

Types of changes

• Bugfix
• Enhancement (a change that doesn't break existing code or deployments)
• Breaking change (a modification that affects current functionality)
• Technical debt (addressing code that needs refactoring or improvements)
• Tests (adding or improving tests)
• Documentation (updates or additions to documentation)
• Maintenance (like dependency updates or tooling adjustments)