Load the folder-access editor lazily to fix multi-minute freeze on large libraries

[janpospis]

Problem

On a large library, loading the folder list freezes for minutes. It is most visible in the admin Folder access editor (the user × folder permission matrix), but it also affects other places that request the full folder list (the file browser's startup default-folder detection, the "move to folder" picker, portals).

Root cause

getFolderList.php (via FolderModel::getFolderList() → getSubfolders()) does a full recursive walk of the whole tree when no folder= is given — O(n) over the entire library. Several callers hit this:

• the folder-access ACL editor (loaded every folder up front and rendered one row per folder),
• AclAdminController::getUserGrants() — calls it with no arguments once per user, so it dominates the matrix load,
• the file browser's startup/move-target folder list, portals, etc.

On a ~50k-folder library, getUserGrants('<user>') measured ~239 s; the walk returned 50,293 folders in 239 s vs 24 top-level folders in 0.11 s.

Changes

• FolderController::getFolderList — when no explicit folder= is given, default to a shallow (top-level) listing instead of the full recursive walk. This fixes every caller that pulled the whole list at once. The full recursive list is still available via &deep=1, and directory navigation (which passes an explicit folder=) is unaffected.
• AclAdminController::getUserGrants — read grants directly from folder_acl.json (the only folders that can carry a grant) instead of walking the tree. ~239 s → ~0.006 s.
• adminFolderAccess.js — the folder-access editor now loads one level at a time (using the shallow folder=<path> mode) with a breadcrumb and per-row drill-in to reach and set permissions on subfolders. In-progress edits are collected into the existing fallback map before navigating, so the batched save still persists grants from every visited level.

Notes

• Callers that previously enumerated the whole tree into a flat list (e.g. the "move to folder" picker) now show the top level; they can request the full list with &deep=1 where needed. On large libraries the full flat list was effectively unusable anyway.
• No new dependencies or schema/config changes. New i18n key open_subfolders (falls back to "Open subfolders").

Testing

Verified on a ~50k-folder local library: the folder-access editor and folder-list loading no longer freeze, drill-in reaches subfolders, and saving persists grants set across multiple levels.