fix(a11y): names that carry what they show; the unused-key check stops covering whole files

- The palette launcher's name replaced its visible "Jump to…" (and the key cap's its "Ctrl K"):
  a voice-control "click Jump to" matched nothing (WCAG 2.5.3). Each name contains its text now.
- SegmentedToggle's `title` was only a tooltip on a non-focusable box — the live search's "how
  many per batch" hint reached no keyboard or screen-reader user. It is the group's accessible
  description, standalone and captioned alike.
- The unused-key contract let a template like `card.${status}` cover its whole namespace, which hid
  five dead keys (deleted). Dynamic keys are matched by shape (`editor.*.hint`); a bare namespace
  shape covers nothing, and the audit/quota namespaces are now held both ways by backendIds.
- CLAUDE.md's "FE control" row matches the lint rule it cites; each no-restricted-syntax entry has
  its reason directly above it.

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

@@ -8,7 +8,6 @@
   "saveForLater": "Save for later",
   "unhide": "Unhide",
   "hide": "Hide",
-  "onlyThisChannel": "Only this channel",
   "resetState": "Reset — clear watch progress/status",
   "thisChannel": "This channel",
   "continueTitle": "Continue where you left off",

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

@@ -1,15 +1,11 @@
 {
   "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."