Two startup cost reductions:
1. Folder enumeration is moved from /api/system/configuration to a new
GET /api/system/folders endpoint. The frontend fetches it only when
the user focuses a folder input field, and caches the result for the
session. On large directory trees this was a noticeable startup delay.
2. The queue array is removed from /api/system/configuration. The queue
is already loaded separately via /api/history/live on mount; including
it in the config response was a double-load that inflated the initial
payload for users with large queues.
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