Improve UX around source folders in Processor (#7101)

# Description of Changes
Adds implicitly defined folders to the list of locations that folder
sources can look in, including the legacy watchedFolder folders, and the
server storage location (if enabled). Also adds a settings UI for
defining the list of allowed folders instead of having to manually edit
`settings.yml` (please excuse the styling, that's the standard styling
of the Processor, hoping it gets fixed by one of the styling PRs).

<img width="888" height="786" alt="image"
src="https://github.com/user-attachments/assets/cf6d0705-adcf-463c-8e80-6901a652068b"
/>

<img width="1103" height="713" alt="image"
src="https://github.com/user-attachments/assets/b7244592-249a-4149-994f-3a2b750f25f9"
/>

app/common/src/main/java/stirling/software/common/model/ApplicationProperties.java

@@ -208,9 +208,10 @@ public class ApplicationProperties {
     public static class Policies {
         /**
          * Absolute directories that policy folder input sources and output sinks may read from or
-         * write to. Empty (the default) disables folder access entirely, so a policy can never be
-         * pointed at an arbitrary server path. Stirling's own config directory is always
-         * off-limits, and folder access is always disabled in SaaS mode regardless of this list.
+         * write to. Empty (the default) disables folder access except to implicitly defined
+         * folders, such as server storage folders (if enabled) and the pipeline watched folders.
+         * Stirling's own config directory is always off-limits, and folder access is always
+         * disabled in SaaS mode regardless of this list.
          */
         private List<String> allowedFolderRoots = new java.util.ArrayList<>();