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/ui/Combobox.tsx

@@ -238,9 +238,7 @@ export default function Combobox({
         {/* The scroll box owns the height cap AND the scroll-fade mask (ref+style on the same element). */}
         <div ref={fade.ref} style={fade.style} className="max-h-64 overflow-y-auto no-scrollbar">
           {loading && filtered.length === 0 ? (
-            <p className="px-2 py-1.5 text-xs text-muted">
-              {t("plex.loading", { defaultValue: "Loading…" })}
-            </p>
+            <p className="px-2 py-1.5 text-xs text-muted">{t("common.loading")}</p>
           ) : filtered.length === 0 ? (
             <p className="px-2 py-1.5 text-xs text-muted">
               {emptyText ?? t("common.noResults", { defaultValue: "No matches" })}
@@ -285,7 +283,7 @@ export default function Combobox({
               })}
               {hiddenCount > 0 && (
                 <p className="px-2 py-1.5 text-2xs text-muted">
-                  {t("plex.filter.moreRefine", {
+                  {t("common.moreRefine", {
                     count: hiddenCount,
                     defaultValue: "{{count}} more — refine with search",
                   })}
@@ -299,7 +297,7 @@ export default function Combobox({
             open in the combobox's focus model. */}
         {matchMode != null && onMatchModeChange && selected.length >= 2 && (
           <div className="mt-1 flex items-center gap-2 border-t border-border px-1 pt-2">
-            <span className="text-2xs text-muted">{t("sidebar.match")}</span>
+            <span className="text-2xs text-muted">{t("common.matchMode")}</span>
             <div className="ml-auto flex gap-1">
               {(["or", "and"] as const).map((m) => (
                 <button