paraglide: variants for section/cluster/mood titles + sort-select labels

[electather]

Part of epic #491. PRD US-012. Review comments on:

• sort-select.tsx:12
• lib/mood-registry.ts:24
• lib/types.ts:41

Goal

Section/cluster/mood titles ("Tonight", "Recently Added", per-mood headers, sort labels) expressed as variants where messages differ only by token (section, cluster, mood, sortKey).

Acceptance criteria

• Cross-feature shared strings identified via single grep pass and consolidated.
• Variants used wherever message differs only by a token.
• Per-locale message count for watchlist+home surface drops ≥30% vs main.
• mood-registry.ts mood labels → paraglide variants keyed on {moodId}.
• sort-select.tsx sort labels → paraglide variants keyed on {sortKey}.
• Verify in browser: each rail/cluster renders correct translated title; sort dropdown options translated.