fix: make Enable animations toggle authoritative over OS reduced-motion

[yanhenrique-dev]

Problem

The Motion card read as Reduce motion (checked = no animation), so activating the checkbox disabled animations — the opposite of what users expect. On top of that, a global prefers-reduced-motion CSS override killed transitions even when the site setting had animations enabled.

Changes

• InterfaceSettings: card now reads Animations / Enable animations with inverted checked state (checked = motion on).
• interface-preferences: added systemPrefersReducedMotion(); missing saved value initializes from the OS, explicit site choice always wins.
• control.css: removed the global prefers-reduced-motion transition kill; toast reduced-motion handling now follows .reduce-interface-motion instead of the OS media query.

Verification

• node --test src/interface-preferences.test.ts: 4 pass
• Manual check: explicit false + OS reduce=true resolves to animations on (site wins); missing value + OS reduce=true resolves to reduced (OS respected by default).