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+# Implementation brief: move Duplicates into library settings
+
+Written for an agent picking this up cold. Read the repo-root `AGENTS.md` and
+`frontend/AGENTS.md` first — this brief assumes both.
+
+**This is a frontend-only change.** The backend already scopes everything by library
+(`GET /books/duplicate-books?library_id=`), so no endpoint, schema or migration is
+involved.
+
+## Where this starts from
+
+The duplicates review screen exists and works. It currently lives at
+
+```
+frontend/src/routes/(root)/(library)/library/[libraryId]/duplicates/
+    +page.server.ts     loads the groups, plus the full Book records merge needs
+    +page.svelte        group cards, "Not duplicates", "Merge…"
+```
+
+and is reached from a **Duplicates entry in the main sidebar**
+(`frontend/src/lib/components/layout/nav-main.svelte`), which is what this change
+removes.
+
+Settings today is a flat, entirely global four-item nav
+(`frontend/src/routes/(root)/settings/+layout.svelte`): Account, Appearance, Libraries,
+Devices. `settings/libraries/+page.svelte` is a single table of every library whose rows
+link *out* to the library itself. **There is nowhere that means "settings for this
+library"** — that is the gap this change fills.
+
+## What to build — option B
+
+Libraries expands in the settings nav. Every library is a sub-item; selecting one swaps
+the pane; Duplicates is a section inside that pane. All libraries and all their sections
+end up one click apart.
+
+```
+/settings/libraries                        the existing table (leave it as the index)
+/settings/libraries/[libraryId]            redirects to the first section
+/settings/libraries/[libraryId]/duplicates the review screen, moved
+```
+
+Suggested files:
+
+| Path | What |
+| --- | --- |
+| `settings/libraries/[libraryId]/+layout.svelte` | Library name, and the section tabs |
+| `settings/libraries/[libraryId]/+page.ts` | `redirect(303, …/duplicates)` |
+| `settings/libraries/[libraryId]/duplicates/+page.server.ts` | Moved verbatim |
+| `settings/libraries/[libraryId]/duplicates/+page.svelte` | Moved verbatim |
+
+Duplicates is the **only** real section today. Build the tab strip so General and Danger
+zone have somewhere obvious to land, but do not invent them now — an empty tab is worse
+than no tab.
+
+## The nav
+
+In `settings/+layout.svelte`, `items` is a flat `as const` array matched on
+`page.route.id`. Libraries needs to render its children beneath it:
+
+```svelte
+{#each libraryState.libraries as library (library.id)}
+  <a href={resolve('/(root)/settings/libraries/[libraryId]/duplicates', {
+        libraryId: String(library.id) })}> … </a>
+{/each}
+```
+
+`getLibraryState()` **is** available under `/settings` — it is set in
+`(root)/+layout.svelte`, above the settings group, and `settings/libraries/+page.svelte`
+already uses it. No new load function is needed to list the libraries.
+
+**Active state is matched on route id, not pathname.** There is a comment in
+`settings/+layout.svelte` explaining why: `resolve()` returns an absolute path on the
+client and a relative one during SSR, so a pathname comparison is false on the server and
+true after hydration, and the highlight flashes in. A nested library item is active when
+the route id matches **and** `page.params.libraryId === String(library.id)` — both, or
+every library lights up at once.
+
+## Things that will bite
+
+1. **Remove the sidebar entry in the same change.** `nav-main.svelte` gained a
+   `Duplicates` item and a `CopyCheck` import when the screen was built. Delete both, and
+   delete the old route directory. Doing the removal and the move together is the point —
+   split across two commits the screen is unreachable in between.
+
+2. **Delete the old route, do not leave it.** Two live copies of a screen that both write
+   is how they drift.
+
+3. **`setBookSelectionState` is not available under `/settings`.** It is set in
+   `(root)/(library)/+layout.svelte`, which the settings group is not inside. This is
+   fine — the duplicates page uses `BookImage` directly, not `book-thumbnail.svelte`, and
+   `MergeBooks` takes its `libraryId` as a prop. **Verify this stays true** if you touch
+   either component; a `getBookSelectionState()` under settings returns `undefined` and
+   fails at the first access, not at import.
+
+4. **Keep `depends('app:duplicate-books')`.** Both the dismiss action and `MergeBooks`
+   call `invalidate('app:duplicate-books')` to make a resolved group leave the screen.
+   Drop it and the page silently stops refreshing. `MergeBooks` also invalidates
+   `app:books`, which is a no-op under settings and should stay that way.
+
+5. **The settings shell is height-constrained.** `settings/+layout.svelte` is
+   `h-[calc(100vh-var(--header-height)-2rem)]` with `overflow-auto` on the content pane.
+   The review screen is a long list of cards — it must scroll *inside* that pane. Its
+   current `mx-auto max-w-5xl` wrapper will want revisiting.
+
+6. **Three levels of nav is option B's known cost.** Nav → library → section, and the
+   pane is narrower than the full-width route the screen was designed against. The group
+   cards are `w-36` covers in a wrapping flex row, so they reflow, but check a group of
+   four at a narrow window before calling it done.
+
+7. **`resolve()` must be a direct call in markup** for `svelte/no-navigation-without-resolve`.
+   Where `nav-main.svelte` computes a url through a variable it carries an
+   `eslint-disable-next-line`; prefer the direct call over inheriting that.
+
+8. **The loader depends on the `?ids=` fix.** `+page.server.ts` fetches full `Book`
+   records with `listBooks({ ids, pageSize })` because the merge workbench needs
+   identifiers, description and publisher, which `DuplicateBookRead` does not carry.
+   advanced_alchemy's stock id filter types that parameter as `list[str]` regardless of
+   config, which made Postgres refuse `bigint = character varying`; the override lives in
+   `backend/src/chitai/services/dependencies.py` (`create_book_filter_dependencies`).
+   If `GET /books?ids=1&ids=2` 500s, that override is missing — do not work around it in
+   the loader.
+
+## Out of scope
+
+The **General** and **Danger zone** sections (rename, path template, read-only, consume
+directory, delete), and any change to the merge workbench itself. The toolbar entry point
+for merge — select 2+ books in the library view — is unrelated and stays where it is.
+
+## Verification
+
+```bash
+cd frontend
+pnpm check     # baseline: 30 errors, 1 warning, 8 files — none of them yours
+pnpm lint      # not clean either; check the files you touched, not the tree
+pnpm build
+```
+
+By hand, with a library that has a duplicate group:
+
+- Settings → Libraries lists every library beneath it; clicking one opens its pane.
+- Duplicates shows the same groups the old route did, and scrolls inside the settings pane.
+- **Not duplicates** removes the group and it stays gone after a reload.
+- **Merge…** opens the workbench, merges, and the group leaves the screen.
+- The main sidebar no longer has a Duplicates entry, and
+  `/library/<id>/duplicates` no longer resolves.
+- A library with no duplicates shows the empty state, not a blank pane.