Exclude course/item leaves from destination picker; ease new-folder UX

The destination picker (Sort Unsorted, Manage Library Move) was
listing individual course/item folders as pickable destinations
whenever they held content course detection doesn't recognize as
video/audio - ebooks, audiobooks in unsupported formats, etc. Add a
leaf-item check scoped to the picker's own category index so these no
longer show up, without touching the shared course-detection heuristic
used elsewhere (Library browsing, search, stats).

Also replace "Create new folder"'s single free-text path field with a
parent-folder picker plus a plain new-folder-name field and a live
"Will create: X/Y" preview, so the resulting path is confirmed before
applying instead of hand-typed blind.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -101,10 +101,18 @@ silently stay blank instead of erroring.
   existing folder, a suggested new subfolder under a broader category match,
   or "needs review" (unchecked by default) when nothing overlaps at all.
   Every row has a folder-picker dropdown (listing every real category in the
-  library, plus "+ Create new folder…" to type a brand-new path) so a wrong
-  or low-confidence guess is one click to correct, and a name field to
-  rename the course's folder in the same move. Nothing on disk moves until
-  you review and hit Apply. Matching ignores common noise (e-learning
+  library, plus "+ Create new folder…") so a wrong or low-confidence guess
+  is one click to correct, and a name field to rename the course's folder
+  in the same move. Creating a new folder means picking its parent from a
+  dropdown and typing just the new folder's own name, with a live "Will
+  create: X/Y" preview so the resulting path is confirmed before applying -
+  the same picker Manage Library's Move action uses. The picker excludes
+  actual course/item folders, not just organizational ones - including a
+  leaf folder holding a single non-video/audio file (an ebook, an
+  audiobook in a format this app doesn't play, etc.) that Library
+  browsing's own course detection wouldn't catch either, since there's no
+  video/audio file to key off. Nothing on disk moves until you review and
+  hit Apply. Matching ignores common noise (e-learning
   platform names, release/distribution-group tags, dates) via a stopword
   list, and beyond that treats a match against a folder's own deliberate
   name as always stronger evidence than a word only borrowed from a