feat(plex): redesign browse tiles — 2-poster montage top, readable label band

The full-bleed 4-poster collage drowned the on-tile text. Split PosterTile in two: the top
half shows up to 2 montage posters, the bottom half is a solid colour-hash band that carries
the label, so the text is always readable. Genre tiles drop the count (just the label now);
collection tiles show the name on one truncated line plus a sub-line of item count · total
runtime. A group with no art falls back to an all-colour tile.

Backend: `/collections` cards gain `total_runtime_s` (a new `runtime_by_key` grouped sum) and
their montage art is capped at 2. Frontend: PosterTile reworked; a `collectionMeta` helper
builds the count·runtime sub-line via the existing formatRuntime.

Verified live: collection and genre tiles render 2 posters over a readable colour band with
the right sub-lines; colour fallback intact for art-less groups.

Gate: tsc/lint/knip/prettier clean, 423 vitest; backend ruff clean, collections tests green
(+ a new art/runtime assertion).

backend/app/plex/query.py

@@ -76,6 +76,23 @@ def top_art_keys(
     return out
 
 
+def runtime_by_key(db, model, lib_ids: list[int], key_col, keys: list[str]) -> dict[str, int]:
+    """Total runtime (sum of duration_s, seconds) of the items under each key in `keys` (a member of the
+    JSONB array `key_col`), as {key: seconds}. ONE grouped query. Used for a collection tile's runtime."""
+    if not lib_ids or not keys:
+        return {}
+    k = func.jsonb_array_elements_text(key_col).label("k")
+    exp = (
+        select(func.coalesce(model.duration_s, 0).label("d"), k)
+        .where(model.library_id.in_(lib_ids), func.jsonb_typeof(key_col) == "array")
+        .subquery()
+    )
+    rows = db.execute(
+        select(exp.c.k, func.sum(exp.c.d)).where(exp.c.k.in_(keys)).group_by(exp.c.k)
+    ).all()
+    return {key: int(total or 0) for key, total in rows}
+
+
 def vector_col(model, lang: str):
     # RANKING vector. In HU we rank by the Hungarian vector; a row matched only via its English vector
     # (see `vector_match`) gets rank 0 and sorts last, which is the intended HU-first ordering.