feat(design): the download, notification and Scheduler action lists (C1) — the rows' "…", the band's and the strip's "…" (E28 S3e3)

A download row keeps play (pause / resume / retry in the queue) and the save link; the editors,
sharing, re-download, the copies and cancel / delete / remove moved into its "…"
(useDownloadActions). A link inside the row keeps the browser's own right-click menu. A
notification row keeps its link and its unread mark and shows its dismiss on hover; an activity
notice's Find in feed and Undo are in its "…". The inbox band keeps Mark all read and puts Clear
everything, now behind a confirm, in its "…". The Scheduler strip keeps Run all and Pause and puts
Purge discovery in its "…". Polled lists key their menus by id (keyOf). ui/menuGroups draws a
rule only between two non-empty groups. Find in feed also drops the tag, search and age filters
that could hide the video it looks for.

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

@@ -3,8 +3,12 @@
   "empty": "You're all caught up.",
   "markAllRead": "Mark all read",
   "clearAll": "Clear all",
+  "clearConfirmTitle": "Clear every notification?",
+  "clearConfirmBody": "Every notification here leaves the list, read or not. They don't come back.",
   "markRead": "Mark read",
   "dismiss": "Dismiss",
+  "rowMenu": "Notification actions",
+  "menu": "Notification center actions",
   "sectionSystem": "System",
   "sectionActivity": "Activity",
   "andMore": "and {{count}} more",