usePromoBackgroundStyles was emitting a string with two background-image
declarations — a url() fallback and an image-set() upgrade. Vue 3's :style
parses string bindings into a normalized object keyed by property name,
collapsing duplicate properties to a single entry. The url() fallback was
silently truncated, leaving browsers that can't fully parse image-set()
(with the type() argument) without any background image.

Fix: the composable now returns an object with two CSS custom properties
(--promo-bg-fallback and --promo-bg-set). Vue keeps both as distinct
entries. Hero.vue's .promo-surface rule declares background-image twice —
once with each var — so browsers walk the natural CSS cascade. The first
declaration always parses; the second overrides only when image-set() is
supported.

Bumps to 1.0.0-beta.29.

Tests: 588 passing (8 in usePromoBackgroundStyles.spec.js rewritten for
the new object contract).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>