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/lib/format.ts

@@ -96,10 +96,16 @@ export function formatDuration(sec: number | null): string {
   return h > 0 ? `${h}:${pad(m)}:${pad(s)}` : `${m}:${pad(s)}`;
 }
 
-/** Compact human runtime ("1h 30m", "45m") from seconds — used for Plex movie/episode lengths.
- * Empty string for a missing/zero duration (nothing to show). */
+/** Compact human runtime ("1h 30m", "45m", "45s") from seconds. Formats BOTH a single item's
+ * length (Plex movies/episodes) and an aggregate total (playlist playtime, on the YouTube and the
+ * Plex side alike) — it is the one renderer for a playlist total, so a total never appears in two
+ * shapes on one screen. Empty string for a missing/zero duration (nothing to show).
+ * The sub-minute tier is load-bearing, not decoration: an aggregate is the only input that can land
+ * between 1 and 59 seconds (a playlist of Shorts), and flooring that to a flat "0m" made a
+ * non-empty playlist report no runtime at all. Keep it, or that case silently regresses. */
 export function formatRuntime(sec?: number | null): string {
   if (!sec) return "";
+  if (sec < 60) return `${Math.floor(sec)}s`;
   const h = Math.floor(sec / 3600);
   const m = Math.floor((sec % 3600) / 60);
   return h ? `${h}h ${m}m` : `${m}m`;