Add a new standalone ContentHero component (template and TS) that
provides a cinematic hero section with backdrop, poster, title,
description, tags/meta/action slots, and a back button event emitter.
The component chooses backdrop or poster as the background, falls back
to a default poster image on load error, and exposes inputs for title,
description, posterUrl, and backdropUrl.
Introduce extensive cinematic styles for the detail view:
- define CSS custom properties for light and dark themes, poster size,
shadows, and color tokens
- implement gradient vignette, layout sizing, and theming via
:host and :host-context(.dark-theme)
- change container sizing behavior to full-height and hide overflow
These changes create a reusable hero UI and themed detail-page styles
to support a Netflix-style streaming detail experience.
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