feat(downloads): editor UI — trim/split/crop clips (phase 2)

VideoEditor modal on a finished Library download: HTML5 <video> scrubber, filmstrip timeline
(lazy server storyboard sprite) with draggable in/out handles, draggable/resizable crop overlay,
per-edit Precise (re-encode) vs Fast (stream-copy) cut toggle, split-into-N fan-out (N trim jobs),
optional clip name. Edited clips show a 'Clip' badge; 'editing' phase gets a % bar. New api
enqueueEdit/downloadStoryboard/storyboardImageUrl + EditSpec type; editor i18n (en/hu/de).

frontend/src/i18n/locales/en/editor.json

@@ -0,0 +1,29 @@
+{
+  "title": "Edit “{{name}}”",
+  "playPause": "Play / pause",
+  "setIn": "Set start",
+  "setOut": "Set end",
+  "in": "Start",
+  "out": "End",
+  "selected": "Selection",
+  "cropOff": "Add crop",
+  "cropOn": "Cropping",
+  "cropReencode": "Cropping always re-encodes (frame-accurate).",
+  "split": "Split into",
+  "accurate": {
+    "label": "Precise cut",
+    "hint": "Frame-accurate. Re-encodes the clip."
+  },
+  "fast": {
+    "label": "Fast cut",
+    "hint": "Instant, but cuts snap to keyframes (±1–2s)."
+  },
+  "nameLabel": "Clip name (optional)",
+  "willReencode": "Will re-encode",
+  "willCopy": "Instant (stream copy)",
+  "create": "Create clip",
+  "createN": "Create {{count}} clips",
+  "created_one": "{{count}} clip added to your downloads",
+  "created_other": "{{count}} clips added to your downloads",
+  "failed": "Couldn’t create the clip."
+}