feat: report skipped duplicates in the upload tray

A book whose files were all already stored settles as skipped rather than done,
naming the book that holds them and offering to add it anyway.

frontend/AGENTS.md

@@ -157,10 +157,12 @@ but take a baseline first, because neither is clean (see below).
 
 Observed in the current tree — don't mistake these for intentional patterns to copy:
 
-- `src/lib/schema/openapi/schema.d.ts` is **stale** — it predates the `BookProgress` rework and has
-  no `BookProgressRead`, so `book.progress` types as `{}`. It is the source of most of the errors
-  `pnpm check` reports; regenerating it should clear them. **Baseline as of 2026-08-11: 37 errors,
-  1 warning, 11 files.** Get your own baseline before assuming an error is yours.
+- `pnpm check` is not clean. **Baseline as of 2026-08-13: 30 errors, 1 warning, 8 files**, most of
+  them in `src/routes/api/[...path]/+server.ts` (see below). Get your own baseline before assuming
+  an error is yours. `src/lib/schema/openapi/schema.d.ts` was regenerated on that date and is
+  current; regenerate it again after any backend API change, with
+  `pnpm exec openapi-typescript http://localhost:8000/schema/openapi.json -o src/lib/schema/openapi/schema.d.ts`
+  against a backend running **your** branch — a stale server silently writes a stale file.
 - `pnpm lint` does not pass either — 131 pre-existing ESLint errors, 35 of them
   `svelte/no-navigation-without-resolve` on plain `href`s, plus ~59 files Prettier would rewrite
   (mostly vendored shadcn components). Check the files you touched, not the whole tree.