Fix NSAllowsArbitraryLoads (Mac App Store)

Part of: #9941

## Problem

All four targets disable App Transport Security with `NSAllowsArbitraryLoads = true`. App Store Review requires ATS to be enabled; a blanket disable causes rejection.

## Affected files (source templates)

- `shell_integration/MacOSX/NextcloudIntegration/FinderSyncExt/Info.plist` (lines 31–35)
- `shell_integration/MacOSX/NextcloudIntegration/FileProviderExt/Info.plist` (lines 15–19)
- `shell_integration/MacOSX/NextcloudIntegration/FileProviderUIExt/Info.plist` (lines 15–19)
- Main app `Info.plist`

## Required change

Set `NSAllowsArbitraryLoads` to `false` in all four plists. Because Nextcloud servers can be self-hosted on plain HTTP, provide a written justification to Apple at submission time explaining that server addresses are user-supplied at runtime and cannot be enumerated at build time.