fix(ui): the platform layer speaks platform words; boxes scale with the text

- PriorityStepper labelled itself with channels.row.* and told every caller
  it orders the Feed; Combobox said plex.loading / plex.filter.moreRefine /
  sidebar.match on the feed sidebar. The generic strings moved to common.*,
  and what a priority MEANS arrives as the stepper's `hint` prop.
  uiI18n.contract.test.ts pins components/ui/ to the platform namespaces —
  the import boundary could never see an i18n key.
- 32 px-frozen box utilities (w-[150px], max-w-[1600px], left-[18px], …)
  became spacing-scale steps (w-37.5, max-w-400, left-4.5): identical at the
  default text size, and now they grow with the Settings slider instead of
  clipping or re-wrapping their own labels. An eslint rule refuses the px
  form beside the older text-[Npx] one, pinned by pxBox.contract.test.ts;
  measured in the browser that every new step is generated at its old px.
- chipLane's import-strip regex ran past a side-effect import into the next
  statement's `from`, hiding a lane between them; it now stops at the
  import's own `;`.

frontend/src/components/PlexPlaylistView.tsx

@@ -572,7 +572,7 @@ export default function PlexPlaylistView({
   ];
 
   return (
-    <div className="mx-auto w-[90%] max-w-[1100px] p-4">
+    <div className="mx-auto w-[90%] max-w-275 p-4">
       <Button variant="glass" size="xs" className="mb-4" onClick={onBack}>
         <ArrowLeft className="h-4 w-4" />
         {t("plex.backToLibrary")}