fix(playlists): one rendering for a playlist total, honest below a minute (E9 R1)

Epic-end deep review, round 1. Six findings survived verification; five of them
were one defect wearing different hats.

formatRuntime floored 1-59s to "0m". Harmless while it only ever formatted a
Plex movie's length, but 12D repurposed it for an aggregate, and a total is the
only input that can land under a minute: a playlist of Shorts reported "0m"
while the rail beside it read "0:45". Fixed in the function, not behind a
`>= 60` guard at each of the seven call sites — the invariant belongs where it
cannot be forgotten. Its contract comment claimed "Plex movie/episode lengths"
and was two epics stale.

The rail rendered the same playlist total as formatDuration (1:23:45) while the
new header rendered formatRuntime (1h 23m) — two shapes of one number, side by
side on one screen. The rail now uses formatRuntime too; formatTotalHours had
already recorded that H:MM:SS is wrong for an aggregate.

The rename input kept min-w-0 without flex-1 in the compacted row, so it was
the only shrinkable child beside a shrink-0 action cluster. The Plex half of
this same epic already had the fix; this restores the symmetry.

plex.browse.items had no plural forms, so a one-item Plex playlist read
"1 items". Pre-existing, but the new header is the call site where count=1 is
ordinary. Both languages get both forms, no key drift.

formatRuntime had no unit tests at all across seven call sites. Added a tier
table plus a sweep asserting no non-zero second ever renders as "0m".

Dropped after 3/3 adversarial refutations: the meta-line segment ordering
(the filtered count is labelled, and count-then-runtime is the convention) and
the null-duration lower bound (the server's own SUM has skipped NULLs since
long before this branch; changing only the client would split them).

frontend/src/components/Playlists.tsx

@@ -630,7 +630,7 @@ export default function Playlists() {
                       if (e.key === "Escape") setRenaming(false);
                     }}
                     aria-label={t("playlists.rename")}
-                    className="min-w-0 text-lg font-semibold bg-surface border border-border rounded-md px-2 py-0.5 focus:outline-hidden focus:ring-2 focus:ring-accent/40"
+                    className="min-w-0 flex-1 text-lg font-semibold bg-surface border border-border rounded-md px-2 py-0.5 focus:outline-hidden focus:ring-2 focus:ring-accent/40"
                   />
                 ) : (
                   <>

frontend/src/components/PlaylistsRail.tsx

@@ -4,7 +4,7 @@ import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { qk } from "../lib/queryKeys";
 import { ListVideo, Pin, Plus, RefreshCw, Youtube } from "lucide-react";
 import { api, type Playlist } from "../lib/api";
-import { formatDuration } from "../lib/format";
+import { formatRuntime } from "../lib/format";
 import { notify } from "../lib/notifications";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
 import { LS, readAccountMerged, writeAccount } from "../lib/storage";
@@ -247,7 +247,7 @@ export default function PlaylistsRail({
                   <div className="text-2xs text-muted">
                     {t("playlists.itemCount", { count: pl.item_count })}
                     {pl.total_duration_seconds > 0 &&
-                      ` · ${formatDuration(pl.total_duration_seconds)}`}
+                      ` · ${formatRuntime(pl.total_duration_seconds)}`}
                   </div>
                 </div>
               </button>