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>

static/js/core/types.js

@@ -297,14 +297,13 @@
 
 /**
  * One item returned by `GET /api/grants/incoming/resources`.
- * Exactly one of `file` / `folder` is populated (indicated by `resource_type`).
+ * `resource_type` discriminates the shape of `resource`.
  * @typedef {Object} SharedWithMeItem
  * @property {ResourceTypeEnum}        resource_type
  * @property {PermissionTypeEnum[]}    permissions   - All permissions the caller holds on this resource.
  * @property {string}                  granted_at    - ISO-8601 timestamp of the earliest grant.
  * @property {string}                  granted_by    - UUID of the user who created the grant.
- * @property {FileItem|undefined}      [file]        - Populated when resource_type === 'file'.
- * @property {FolderItem|undefined}    [folder]      - Populated when resource_type === 'folder'.
+ * @property {FileItem|FolderItem}     resource      - Full resource details; shape follows resource_type.
  */
 
 /**

static/js/views/sharedWithMe/sharedWithMeView.js

@@ -150,8 +150,8 @@ const sharedWithMeView = {
         const ownerMap = new Map();
 
         for (const item of items) {
-            if (item.resource_type === 'folder' && item.folder) {
-                const f = item.folder;
+            if (item.resource_type === 'folder') {
+                const f = /** @type {FolderItem} */ (item.resource);
                 folders.push(
                     /** @type {FolderItem} */ ({
                         id: f.id,
@@ -168,8 +168,8 @@ const sharedWithMeView = {
                     })
                 );
                 ownerMap.set(f.id, item.granted_by);
-            } else if (item.resource_type === 'file' && item.file) {
-                const f = item.file;
+            } else if (item.resource_type === 'file') {
+                const f = /** @type {FileItem} */ (item.resource);
                 files.push(
                     /** @type {FileItem} */ ({
                         id: f.id,