feat(api): cursor listing contract — PageCursor trait + resource field

- Add src/application/dtos/cursor.rs with three shared types:
  · PageCursor trait  — default base64url+JSON encode/decode; one bare
    impl line per cursor struct
  · CursorQuery struct — standard limit/cursor/sort_by query params with
    limit_clamped() and decode_cursor<C>() helpers; compose via flatten
  · CursorListResponse<T> — standard {items, next_cursor?} envelope with
    from_oversized() and with_cursor() builders

- Migrate GrantCursor to impl PageCursor (remove duplicate encode/decode)

- Update GET /api/grants/incoming/resources:
  · SharedWithMeQuery now embeds CursorQuery via #[serde(flatten)]
  · Replace file/folder nullable pair with ResourceContentDto (untagged
    enum) under a single always-present 'resource' field
  · SharedWithMeDto is now a type alias for CursorListResponse<SharedWithMeItemDto>
  · Handler uses q.paging.limit_clamped() and decode_cursor<GrantCursor>()

- Add docs/architecture/resource-listing.md — authoritative contract for
  all listing endpoints (cursor design, SQL keyset WHERE, sort_by naming,
  Rust + JS skeletons, compliance table, migration guide)

- Register doc in VitePress sidebar and architecture index

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/architecture/index.md

@@ -71,4 +71,5 @@ src/
 ## Further Reading
 
 - [Caching Architecture →](/architecture/caching)
+- [Resource Listing API →](/architecture/resource-listing)
 - [Storage Quotas →](/architecture/storage-quotas)