web: overlay a draggable histogram on the photo

The panel in Lightroom is a running read of the render, so this one reads
the preview blob itself: one downscaled 320px canvas pass bins 256
values per channel, and four SVG paths draw them — the grey luma fill
with the three channel curves screened over it, from absolute black to
absolute white. Dragged by its header, clamped inside the photo, parked
top-right on first paint, and dismissed either from its own frame or from
the toolbar button. It steps aside while the crop frame is up.

docker/frontend/src/i18n/en.ts

@@ -27,6 +27,10 @@ export const en: Dict = {
   'drop.compare': 'Compare with original picture',
   'drop.demoLimit': 'Demo build: exports are capped at {px}px and carry a watermark.',
 
+  'hist.title': 'HISTOGRAM',
+  'hist.drag': 'Drag to move',
+  'hist.hide': 'Hide the histogram',
+
   'tab.presets': 'PRESETS',
   'tab.favorited': 'FAVORITED',
   'tab.photos': 'MY PHOTOS',

docker/frontend/src/i18n/vi.ts

@@ -34,6 +34,11 @@ export const vi = {
   'drop.compare': 'So với ảnh gốc',
   'drop.demoLimit': 'Bản demo: ảnh xuất ra tối đa {px}px và có watermark.',
 
+  // The histogram overlay. 'HISTOGRAM' is the word Vietnamese photographers use.
+  'hist.title': 'HISTOGRAM',
+  'hist.drag': 'Kéo để di chuyển',
+  'hist.hide': 'Ẩn histogram',
+
   'tab.presets': 'PRESETS',
   'tab.favorited': 'FAVORITED',
   'tab.photos': 'ẢNH CỦA TÔI',