fix(ui): the card's channel name is a real link too; call sites the bug hunt left untested now have tests

Deep-review R6 — the small items:

- The video card's channel name was still a <button> (pre-existing): ctrl/middle-click did nothing
  there while the title above it and the player's channel name honoured them. It renders through
  ChannelNameLink now, which grew a forwarded ref (the clamp measurement), pass-through attributes,
  a `tooltip`, and an opt-in `stopPropagation` so no click reaches the card's row.
- ChannelNameLink rendered an empty, still-tabbable <a> for a channel with no title yet (a stub first
  seen through a live search; pre-existing at this site). It falls back to the translated "Channel".
- The feed's SortKey→SortOption mapping existed twice in Feed.tsx; one `feedSortOptions` in
  feedSort.ts serves both modes (DIRECTIONLESS is module-private again).
- The backfill refresh's comment claimed a one-page cost. It refetches every loaded page of the
  grid, and that is the right price: backfilled videos are older, so they land below the head.

Tests for call sites that could be deleted with every lane green: the channel page running the
backfill refresh (knip had been blinded by the hook test's import), Feed handing the grid the live
results in the chosen key AND direction, the card using the link. Plus the alt-click arm of the shared
click predicate, and a plex filter-chip test that claimed it made no request while firing a real one.
Verified on localdev: the feed's channel names are anchors (same 14 px muted look), a plain click
opens the channel in-app. 7 mutations, each red on its assertion.

frontend/src/lib/feedSort.yt.test.ts

@@ -1,5 +1,11 @@
 import { describe, it, expect } from "vitest";
-import { sortYtResults, YT_SORT_KEYS, SORT_KEYS } from "./feedSort";
+import {
+  feedSortOptions,
+  sortYtResults,
+  SORT_DEFAULT_DIR,
+  YT_SORT_KEYS,
+  SORT_KEYS,
+} from "./feedSort";
 import type { Video } from "./api";
 
 // The live-YouTube results view shipped with no sort control at all: the provider's order was the
@@ -111,3 +117,17 @@ describe("sortYtResults", () => {
     expect(titles(input)).toEqual(titles(RESULTS));
   });
 });
+
+describe("feedSortOptions", () => {
+  // The library feed and the live results used to build their options from two copies of this
+  // mapping; one helper means a key cannot get a different default direction in one of the modes.
+  it("gives directional keys their default direction and marks shuffle/relevance directionless", () => {
+    const opts = feedSortOptions(["relevance", "date", "duration", "shuffle"], (k) => `label:${k}`);
+    expect(opts.map((o) => [o.key, o.label, !!o.directionless, o.defaultDir])).toEqual([
+      ["relevance", "label:relevance", true, undefined],
+      ["date", "label:date", false, SORT_DEFAULT_DIR.date],
+      ["duration", "label:duration", false, SORT_DEFAULT_DIR.duration],
+      ["shuffle", "label:shuffle", true, undefined],
+    ]);
+  });
+});