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/PlexBrowse.tsx

@@ -469,7 +469,7 @@ export default function PlexBrowse() {
 
   if (sub.view.kind === "discover") {
     return (
-      <div className="p-4 max-w-[1600px] mx-auto">
+      <div className="p-4 max-w-400 mx-auto">
         <PlexDiscover
           scope={scope}
           onOpenCard={onCard}
@@ -484,25 +484,25 @@ export default function PlexBrowse() {
   // playlist tile opens that playlist; a collection tile filters the title grid by that collection.
   if (sub.view.kind === "playlists") {
     return (
-      <div className="p-4 max-w-[1600px] mx-auto">
+      <div className="p-4 max-w-400 mx-auto">
         <PlexPlaylistsBrowse onOpen={(id) => sub.open({ kind: "playlist", id })} />
       </div>
     );
   }
   if (sub.view.kind === "collections") {
     return (
-      <div className="p-4 max-w-[1600px] mx-auto">
+      <div className="p-4 max-w-400 mx-auto">
         <PlexCollectionsBrowse onPick={(id) => sub.open({ kind: "collection", id })} />
       </div>
     );
   }
 
   return (
-    <div className="p-4 max-w-[1600px] mx-auto">
+    <div className="p-4 max-w-400 mx-auto">
       {/* The title count is fixed chrome — it portals into the shell's fixed band so it stays put
           while only the poster grid scrolls. */}
       <PageToolbar>
-        <div className="px-4 pt-2 pb-1 max-w-[1600px] mx-auto">
+        <div className="px-4 pt-2 pb-1 max-w-400 mx-auto">
           <div className="flex items-center gap-1.5">
             <ItemCount count={browseQ.isLoading ? undefined : total} />
             {/* Watch state — a top-level toolbar pill next to the count (moved out of the rail),
@@ -557,7 +557,7 @@ export default function PlexBrowse() {
                   </div>
                 )}
                 <div className="min-w-0">
-                  <div className="text-sm font-medium truncate max-w-[150px] group-hover/person:text-accent">
+                  <div className="text-sm font-medium truncate max-w-37.5 group-hover/person:text-accent">
                     {p.name}
                   </div>
                   <div className="text-2xs text-muted">