feat(library): the single view walks the shelf from the picture itself, by arrow and by finger

SINGLE is where a roll is read one frame at a time, and the step from one frame
to the next was a trip to the strip below the picture and back. The stage now
carries the pair itself, at the edges the eye already reads as "there is more
this way", dimmed at the ends of the roll rather than taken away so the pair does
not jump about. The press stops at the arrow: the zoom, the pan and the open
under it are the picture's.

The phone's full-screen view has no room for a pair, so the same step is the
reader's own finger. A drag that travelled sideways is a step the way the list is
read; one that wandered up and down, and a tap in place, are left alone.

Both walk the list on screen — filters and sort included — so what is drawn after
the step is what stood beside it.

Verified against a local preview with a seeded shelf: the arrows appear, the left
one is dimmed on the newest frame and the right one on the oldest, and a left
swipe on the phone moves to the next frame and back.

docker/frontend/src/i18n/en.ts

@@ -278,6 +278,8 @@ export const en: Dict = {
   'lib.colWidth': 'Drag to set the width of the folder column (arrow keys when focused)',
   'lib.viewPreview': 'SINGLE',
   'lib.viewGrid': 'ALL THUMBS',
+  'lib.prev': 'Previous frame',
+  'lib.next': 'Next frame',
   'lib.subsShow': 'SHOW SUBFOLDERS',
   'lib.subsHide': 'THIS FOLDER ONLY',
   'lib.subsHint': 'Frames in subfolders: the whole branch under the open folder, or only what that folder holds itself.',

docker/frontend/src/i18n/vi.ts

@@ -289,6 +289,8 @@ export const vi = {
   'lib.colWidth': 'Kéo để chỉnh độ rộng cột thư mục (mũi tên trái/phải khi đang chọn)',
   'lib.viewPreview': 'MỘT ẢNH',
   'lib.viewGrid': 'TOÀN BỘ',
+  'lib.prev': 'Ảnh trước',
+  'lib.next': 'Ảnh sau',
   'lib.subsShow': 'HIỆN CẢ THƯ MỤC CON',
   'lib.subsHide': 'CHỈ THƯ MỤC ĐANG CHỌN',
   'lib.subsHint':