Tighten Sort Unsorted matching; consolidate file management tools

Fix Sort Unsorted picking a wrong destination on words that are common
clutter across many categories (a creator's name, generic marketing
filler) by ranking matches on a folder's own deliberate name over
incidental sibling-course words, and downweighting borrowed words that
recur across many categories. Add a folder-picker/create-new/rename UI
to override any proposal.

Turn /unsorted into a general File Management page: move Refresh
Library, Bulk Rename, and Manage Library (hide/rename) here from
Settings, add a Move action to Manage Library using the same picker,
and add Duplicate Courses and Clean Up Stale References tools. Add a
footer link to the page from the dashboard.

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

README.md

@@ -88,19 +88,52 @@ silently stay blank instead of erroring.
 - Cover art: uses a manually-placed `cover`/`folder`/`thumbnail`/`thumb`/
   `poster` image if a course has one, otherwise auto-generates one via
   `ffmpeg` from a frame of the course's first video
-- Hide courses/folders from the browser without touching anything on disk;
-  bulk-select to hide or queue several at once
-- Bulk rename across course/folder names (find & replace)
-- **Sort Unsorted** ([/unsorted](templates/unsorted.html), linked from
-  Settings): drop new/incoming courses into an `Unsorted` folder at the
-  library root, then scan proposes a destination for each one by keyword
+- **File Management** ([/unsorted](templates/unsorted.html), linked from
+  Settings and from a footer link on the dashboard): a dedicated page for
+  everything that touches files on disk, since it needs more room than a
+  Settings card and warrants review before anything actually changes.
+  - *Sort Unsorted*: drop new/incoming courses into an `Unsorted` folder at
+  the library root, then scan proposes a destination for each one by keyword
   overlap against the *existing* category tree - read fresh from disk every
   scan, so it adapts to whatever folders you actually have rather than any
   hardcoded subject list. Three outcomes per item: a confident match to an
   existing folder, a suggested new subfolder under a broader category match,
   or "needs review" (unchecked by default) when nothing overlaps at all.
-  Every destination is editable before applying, and nothing on disk moves
-  until you review and hit Apply.
+  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
+  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
+  sibling course's title - and among those borrowed words, downweights
+  ones that recur across many categories (a prolific creator's name, etc.)
+  so they can't outvote a genuinely specific word just by sharing more of
+  them.
+  - *Refresh Library*: manually bypasses the 5-minute filesystem-scan cache,
+  for when files were added/removed directly on disk.
+  - *Bulk Rename*: find & replace across every course/folder name in the
+  library at once, with a per-match preview and the ability to drop
+  individual matches before applying.
+  - *Manage Library*: hide courses/folders from the browser without
+  touching anything on disk (hiding a folder hides everything inside it;
+  bulk-select elsewhere to hide or queue several at once), rename a
+  course/folder in place, or **move** one anywhere else in the library via
+  the same folder-picker/create-new-folder UI Sort Unsorted uses - handy
+  for correcting a bad auto-sort later or just reorganizing.
+  - *Duplicate Courses*: scans every course in the library (Unsorted
+  included) for names that look like the same thing filed twice, using the
+  same tokenizer as Sort Unsorted so a platform-name or release-tag
+  difference doesn't hide a real duplicate. Groups by similarity (union of
+  any two courses over the threshold, transitively) rather than showing
+  raw pairs; each course in a group gets a one-click Hide.
+  - *Clean Up Stale References*: finds entries in the hidden-paths list,
+  Next Up queue, or Recently Viewed history that point at a path no longer
+  on disk - normally from renaming/moving/deleting a course directly on
+  the NAS instead of through the app (doing it through the app already
+  keeps these in sync). Review individually or clear them all at once;
+  course files themselves are never touched.
 
 **Dashboard**
 - Library-wide stats (courses, lessons completed, time watched, time
@@ -138,8 +171,8 @@ silently stay blank instead of erroring.
   actual color distance), or a fully custom accent color, corner radius,
   and card style
 - Font, text size, page width, and spacing density
-- Default library path override + manual "Refresh Library" (bypasses the
-  5-minute filesystem-scan cache)
+- Default library path override (Refresh Library and Bulk Rename live on the
+  File Management page - see above)
 
 **Backup & integrations**
 - One-click backup export and restore: settings, hidden-path choices, the