* Drop the "No strategies in this folder" line under folders

When a library view held only folders, a grey "No strategies in this
folder" floated mid-screen. It showed at the root, which isn't a folder,
and next to folders holding hundreds of strategies, so it read as wrong.
The folder cards already show what's there, and a view with nothing in
it keeps its own empty state.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

* Test the library's folders-only and empty views

A view holding only folders renders just the folder cards, and a view
with nothing in it offers New Strategy. The first fails on the old
placeholder.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>