feat: unify image plugin features (backgroundImage, style, refs) across all HTML-based plugins

[isamu]

Problem

HTML-based image plugins have inconsistent feature support. Features implemented in some plugins are missing in others, even though the infrastructure (resolveCombinedStyle, resolveImageRefs, etc.) already exists.

Current Feature Matrix

Feature	chart	markdown	mermaid	textSlide	html_tailwind	slide
backgroundImage	❌	✅	✅	✅	❌	✅ (branding)
style (CSS)	❌	✅	✅	✅	❌	❌
image:name refs	❌	✅	❌	❌	✅	✅ (imageRef)
movie:name refs	❌	✅	❌	❌	✅	❌

Planned Changes (Priority order)

1. chart: add backgroundImage — call resolveCombinedStyle, add backgroundImage to schema
2. chart: add style (CSS) — add style field to schema, pass to resolveCombinedStyle
3. mermaid: add image:name refs — call resolveImageRefs on generated HTML
4. textSlide: add image:name refs — call resolveImageRefs on markdown content
5. slide: add movie:name refs — resolve movie:name in slide HTML
6. chart: add image:name refs — resolve image:name in chart HTML (for background references)
7. mermaid: add movie:name refs — resolve movie:name in mermaid HTML

Background Video Support

Currently backgroundImage only supports static images via CSS background-image. Add support for video backgrounds using HTML <video> element positioned behind content — useful for immersive presentations with movie scenes as backgrounds.

User Prompt

• chartやmarkdownでreferenceを使ったbg指定ができるようにしたい
• HTMLベースのプラグインで機能の抜けをチェックして統一
• bgに動画も入れられるようにしたい（イマーシブに）