mood-cluster.tsx: use existing text-size design tokens + apply suggested className

[electather]

Part of epic #491. Review comments:

• apps/client/src/features/watchlist/components/sections/mood-mosaic/mood-cluster.tsx:51 — apply suggestion:

  <div className="mb-1 font-mono text-xs uppercase tracking-[0.16em] text-muted-foreground/70">

• apps/client/src/features/watchlist/components/sections/mood-mosaic/mood-cluster.tsx:54 — "use existing variables for text size."

Acceptance criteria

• Suggested className applied at :51.
• Text size at :54 uses existing design token (e.g. shared text-size class), not bespoke value.
• Typecheck passes; visual unchanged.