Currently Borrowed page: checkbox multi-select + bulk Return with per-item condition comments

# Why?

**As a** user returning multiple borrowed items at once
**I want** the Currently Borrowed page to work like the items table (checkbox multi-select) with a bulk Return action
**So that** returning several items doesn't require repeating a single-item return flow for each one.

# What?

Rework the Currently Borrowed view (dashboard tab, Phase 3/#335) to match the items-table selection mechanism (row checkboxes + the tri-state header checkbox from #354):
- Add per-row checkboxes to the Currently Borrowed list.
- Add a "Return" button (e.g. at the bottom of the list) enabled when at least one row is selected.
- Clicking Return navigates to a returns confirmation page pre-populated with the selected items, each individually deselectable, matching the batched-return-by-reservation flow already built in Phase 7 (#339) — reuse that flow/page rather than building a second one, since Phase 7 already added per-item condition-notes fields and a `ReturnGroup`-style read API.

Clarify during triage whether this should literally be "select any subset of currently-borrowed items across different borrow groups and return them together" (broader than Phase 7's single-borrow-group batching) or just apply Phase 7's existing per-group flow more consistently in the UI.

# Testing

Playwright e2e: borrow 2+ items, select a subset from Currently Borrowed, return them with condition comments, confirm they no longer appear in Currently Borrowed and the comments appear in History.

# Additional context

Depends on #354 (checkbox pattern) and Phase 7's batched-return-with-condition-notes work (#339, merged).