test(channels): cover applySyncFilter + DataTable fixedLayout

- applySyncFilter: the fully/needsFull/all predicate the top chips drive
  (moved off the retired Sync column) — guards which channels show.
- DataTable fixedLayout: asserts the colgroup + table-fixed appear only
  when opted in, with widths on fixed columns and none on fluid ones.

frontend/src/components/DataTable.fixedLayout.dom.test.tsx

@@ -0,0 +1,46 @@
+// @vitest-environment jsdom
+//
+// The R14 zoom-fit foundation: DataTable's opt-in `fixedLayout` renders a <colgroup> and puts the
+// table into `table-layout: fixed`, so columns with a width keep it and the rest (fluid text columns)
+// split the remainder + truncate instead of pushing the table wider than its container. Off by
+// default so existing content-sized tables are untouched.
+import { describe, it, expect, afterEach, vi } from "vitest";
+import { render, cleanup } from "@testing-library/react";
+import DataTable, { type Column } from "./DataTable";
+
+vi.mock("react-i18next", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("react-i18next")>();
+  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+});
+
+afterEach(cleanup);
+
+type Row = { a: string; b: string };
+const columns: Column<Row>[] = [
+  { key: "a", header: "A", width: "5rem", render: (r) => r.a },
+  { key: "b", header: "B", render: (r) => r.b }, // fluid — no width
+];
+const rows: Row[] = [{ a: "1", b: "hello" }];
+
+describe("DataTable fixedLayout", () => {
+  it("renders a width-bearing colgroup and table-fixed only when fixedLayout is set", () => {
+    const { container } = render(
+      <DataTable rows={rows} columns={columns} rowKey={(r) => r.a} fixedLayout />,
+    );
+    const table = container.querySelector("table");
+    expect(table?.className).toContain("table-fixed");
+
+    const cols = container.querySelectorAll("colgroup col");
+    expect(cols).toHaveLength(2);
+    expect((cols[0] as HTMLElement).style.width).toBe("5rem"); // fixed column keeps its width
+    expect((cols[1] as HTMLElement).style.width).toBe(""); // fluid column: no width, splits remainder
+  });
+
+  it("omits the colgroup and stays auto-layout by default", () => {
+    const { container } = render(
+      <DataTable rows={rows} columns={columns} rowKey={(r) => r.a} />,
+    );
+    expect(container.querySelector("colgroup")).toBeNull();
+    expect(container.querySelector("table")?.className).not.toContain("table-fixed");
+  });
+});

frontend/src/components/channelColumns.dom.test.tsx

@@ -0,0 +1,56 @@
+// @vitest-environment jsdom
+//
+// applySyncFilter is the sync-status pre-filter the top chips drive (R14 retired the Sync column and
+// folded its status into a Stored charm). It replaced a column `filter.test`, so it carries the exact
+// "fully" | "needsFull" | "" vocab + predicate — asserted here so a regression can't silently change
+// which channels the top chips show.
+import { describe, it, expect } from "vitest";
+import { applySyncFilter } from "./channelColumns";
+import { type ManagedChannel } from "../lib/api";
+
+const ch = (over: Partial<ManagedChannel>): ManagedChannel => ({
+  id: "c",
+  title: null,
+  handle: null,
+  thumbnail_url: null,
+  subscriber_count: null,
+  video_count: null,
+  description: null,
+  stored_videos: 0,
+  last_video_at: null,
+  total_duration_seconds: 0,
+  count_normal: 0,
+  count_short: 0,
+  count_live: 0,
+  priority: 0,
+  hidden: false,
+  deep_requested: false,
+  deep_in_queue: false,
+  tag_ids: [],
+  details_synced: false,
+  recent_synced: false,
+  backfill_done: false,
+  ...over,
+});
+
+const rows = [
+  ch({ id: "fully", recent_synced: true, backfill_done: true }),
+  ch({ id: "recentOnly", recent_synced: true, backfill_done: false }),
+  ch({ id: "none", recent_synced: false, backfill_done: false }),
+];
+
+describe("applySyncFilter", () => {
+  it("returns every row for the empty/all value (and unknown values)", () => {
+    expect(applySyncFilter(rows, "").map((r) => r.id)).toEqual(["fully", "recentOnly", "none"]);
+    expect(applySyncFilter(rows, undefined)).toHaveLength(3);
+    expect(applySyncFilter(rows, "bogus")).toHaveLength(3);
+  });
+
+  it("'fully' keeps only channels with recent uploads AND full history in", () => {
+    expect(applySyncFilter(rows, "fully").map((r) => r.id)).toEqual(["fully"]);
+  });
+
+  it("'needsFull' keeps every channel whose back-catalogue is not done", () => {
+    expect(applySyncFilter(rows, "needsFull").map((r) => r.id)).toEqual(["recentOnly", "none"]);
+  });
+});