feat: IIIF Collection browsing and navigation

[nikazzio]

Summary

Support IIIF Collection manifests, allowing users to browse a library's entire catalog, navigate nested collections, and select individual manifests for download/study.

Motivation

Many digital libraries (BnF, BSB, BL, Vatican) expose IIIF Collections that list hundreds or thousands of manuscripts. Today the app only accepts individual manifest URLs — users must find each manifest externally. Collection browsing unlocks a natural discovery workflow.

Proposed approach

• Detect Collection type in the manifest/discovery resolution pipeline.
• Render a browsable tree/list UI: Collection → sub-collections → manifests.
• Allow selecting one or multiple manifests for download or import to Library.
• Support both IIIF Presentation API 2.1 Collections and 3.0 Collections.
• Lazy-load nested collections on expand (don't fetch the entire tree upfront).

Acceptance criteria

• Pasting a Collection URL shows the collection browser instead of a single manifest
• Nested collections are navigable (expand/collapse)
• Individual manifests can be selected and opened in Studio
• Batch import: select N manifests → add all to Library
• Works with both Presentation API 2.x and 3.0 collections
• Graceful fallback if a collection is too large (pagination / virtual scroll)

Technical notes

• Collection detection can go in discovery.py resolver pipeline.
• IIIF Presentation 3.0 collection: type: "Collection" with items[].
• IIIF Presentation 2.1 collection: @type: "sc:Collection" with collections[] and manifests[].
• Consider caching fetched collection metadata in SQLite.