Web redesign Phase 5/8: Multi-select return flow + Batches view

# Why?

**As a** user with multiple borrowed items
**I want** a multi-select return flow (instead of returning one item at a time) and a read-only view of batches
**So that** returning several items I borrowed together doesn't require repeating a single-item flow, matching our internal Retool RMS app's "Return Items" table and Batches tab.

# What?

Phase 5 of an 8-phase redesign of `web/` modeled after our internal Retool RMS app (`https://a2nseattle.retool.com/p/rms-alpha/main`). Can proceed in parallel with Phase 4 once Phase 1's design system has landed; no dependency on Phase 2 or 3.

Scope:
- Return-items page matching Retool's "Return Items" table: list of currently-borrowed items, multi-select, a shared notes field, and a single **"Confirm Return"** action.
- Reuses the existing `returnItem` API, which already accepts `ids: string[]` — this phase is about exposing that as a multi-select UI, not a backend change.
- **Batches** tab/page: read-only view over `BatchSchema`/`GetBatch`, both of which already exist backend-side (no new backend read needed).

# Testing

Per this repo's testing policy: Jest/RTL for the multi-select return UI, Playwright e2e for the multi-item return flow, and a smaller RTL/e2e check for the read-only Batches view rendering correctly.

# Additional context

Part of an 8-phase `web/` redesign series (#333 Phase 1, #334 Phase 2, #335 Phase 3, #336 Phase 4). Phase 7 (batched return with condition notes) supersedes/extends this phase's return flow with reservation-based grouping and per-item condition notes — expect this phase's return UI to be the starting point Phase 7 builds on, not final.

Die Arbeit umfasst die Erstellung einer neuen Rückgabeseite mit Mehrfachauswahl und einer schreibgeschützten Batches-Ansicht im Verzeichnis web/. Beginne mit der Untersuchung des vorhandenen returnItem-API-Aufrufs und der Typen BatchSchema/GetBatch. Sieh dir das Designsystem aus Phase 1 (#333) und den aktuellen Ablauf für die Rückgabe eines einzelnen Elements an. Schreibe Jest/RTL-Tests für die UI-Komponenten und Playwright-Tests für den End-to-End-Ablauf der Rückgabe mehrerer Elemente. „Done“ bedeutet, dass beide neuen Seiten implementiert und getestet sind und der Funktionalität der referenzierten Retool-App entsprechen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.