docs(admin): hash conflicts page — the duplicate-content queue

Sixth admin page. Documents what creates conflict groups (scanner at
scan time + the hash backfill retroactively), the group card anatomy
(count badge, library + short SHA-256, per-copy rows with usage summary
highlighted when that copy holds reading data), and both resolutions
with verified semantics: Keep this copy (reparents the other copies'
children into the kept copy, deletes their catalog rows, records the
resolver — files on disk never touched, consistent with platform-wide
delete semantics) and Keep both (mark resolved, stop flagging, nothing
merged). Glossary gains Keep both and Keep this copy.

docs/developer/ui-controls.md

@@ -284,6 +284,14 @@ label: User Access
 where: Admin → Libraries → a library's panel → section heading
 does: Per-user visibility checkboxes — checked (default) shows the library to that user; unchecked hides it everywhere for them, enforced in the database.
 
+label: Keep both
+where: Admin → Hash Conflicts → a conflict group card
+does: Records the resolution and stops flagging the group; every copy stays, nothing merged or removed. For intentional duplicates.
+
+label: Keep this copy
+where: Admin → Hash Conflicts → a copy row
+does: Keeps that copy, folds the other copies' reading data into it, and removes their catalog rows. Files on disk are never touched.
+
 label: Resume rewind
 where: audio player → player settings; profile settings
 does: Rewinds playback a configured few seconds when you resume.