web: browse the catalogue as a tree of folders

The library page was a flat wall of thumbnails. It now reads like the admin
pictures tab: a folder tree on the left, the picked node's frame on the stage,
that node's frames in the strip below, and a chip pair to swap the stage for a
wall of every thumbnail in the node.

Folders are walked recursively (dot/@-prefixed names skipped, an unreadable
subfolder is dropped rather than failing the scan), so a roll with a dated
subfolder keeps both levels. Node counts include the subfolders underneath.

docker/frontend/src/i18n/en.ts

@@ -225,7 +225,7 @@ export const en: Dict = {
   'photos.failed': 'That did not work: {msg}',
   'photos.noLabels': 'No strip labels yet',
 
-  'lib.title': 'Library',
+  'lib.title': 'LIBRARY',
   'lib.hint':
     'Pick a photo folder on this machine. Nothing is uploaded — the page only remembers where the folder is and keeps one thumbnail per frame.',
   'lib.add': 'ADD FOLDER',
@@ -233,7 +233,9 @@ export const en: Dict = {
   'lib.rescan': 'RESCAN',
   'lib.remove': 'REMOVE',
   'lib.reconnect': 'GRANT ACCESS AGAIN',
-  'lib.all': 'ALL',
+  'lib.tree': 'FOLDERS',
+  'lib.viewPreview': 'SINGLE',
+  'lib.viewGrid': 'ALL THUMBS',
   'lib.open': 'Open in the studio',
   'lib.edited': 'edited',
   'lib.size': '{mb} MB',
@@ -250,7 +252,7 @@ export const en: Dict = {
   'nav.admin': 'Admin',
   'nav.photos': 'My photos',
   'nav.profile': 'Profile',
-  'nav.library': 'Library',
+  'nav.library': 'LIBRARY',
   'nav.studio': 'Studio',
   'adm.title': 'Strip moderation',
   'adm.subtitle': 'Photos contributed by users. Delete one to pull it off the landing page.',