Add delete and ignore actions to Duplicate Courses

Delete permanently removes a course from disk (confirmed with the full
path first) and cleans up any Hidden/Next Up/Recently Viewed
references to it. Ignore marks a specific pair as confirmed-not-
duplicates so it stops resurfacing in scans, without suppressing
either course's other matches; ignored pairs are listed and reversible
under "Ignored matches," and now travel with backup/restore.

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

README.md

@@ -128,7 +128,14 @@ silently stay blank instead of erroring.
   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.
+  raw pairs; each course in a group gets a one-click Hide, or a **Delete**
+  that permanently removes it from disk (the one destructive action in the
+  whole app - confirmed with the full path before it runs). A group can
+  also be marked **"Not a duplicate"** if the match is wrong, which
+  excludes that specific pair from future scans without touching anything
+  else that happens to match one of those courses; ignored pairs are
+  listed (and reversible) under "Ignored matches," and persist through
+  backup/restore alongside hidden paths and Next Up.
   - *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
@@ -177,8 +184,9 @@ silently stay blank instead of erroring.
 
 **Backup & integrations**
 - One-click backup export and restore: settings, hidden-path choices, the
-  Next Up queue, recent-view history, Outline config, and every course's
-  progress/notes as a zip (not the course files themselves). Restore
+  Next Up queue, recent-view history, ignored-duplicate pairs, Outline
+  config, and every course's progress/notes as a zip (not the course files
+  themselves). Restore
   overwrites current data and needs a matching course folder to already
   exist for each course's progress to land - it's a "put my data back"
   action, not a merge
@@ -197,6 +205,7 @@ Everything under `OFFLINEU_DATA_DIR` (app-wide, not tied to a course):
 | `hidden_paths.json`    | Courses/folders curated out of the browser |
 | `next_up.json`         | The Next Up queue, in order                |
 | `recent_views.json`    | Cross-course "Recently Viewed" history     |
+| `ignored_duplicates.json` | Course-path pairs confirmed not duplicates |
 | `outline_config.json`  | Outline API token/collection mapping       |
 
 Per-course, written inside the course's own folder on the library volume: