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.dom.test.tsx

@@ -137,7 +137,7 @@ describe("Combobox", () => {
     wrap(<Combobox label="Studio" options={many} selected={[]} onChange={vi.fn()} />);
     fireEvent.focus(screen.getByRole("combobox"));
     expect(screen.getAllByRole("option", { hidden: true }).length).toBe(50);
-    expect(screen.getByText("plex.filter.moreRefine")).toBeTruthy(); // 10 hidden behind search
+    expect(screen.getByText("common.moreRefine")).toBeTruthy(); // 10 hidden behind search
   });
 
   it("Enter commits the pointer-highlighted option, not the arrow-active one", () => {