fix(reader): inject reader-fonts.css into iframe, remove foliate-themes.css

Fonts weren't loading because the paginator renders inside a sandboxed
iframe. @font-face declarations in the parent page's CSS are invisible
to the iframe's document. Even injecting @font-face rules via
setStyles() may not trigger font loading in sandboxed iframes.

Fix: inject a <link> to reader-fonts.css directly into the iframe's
document on each section load, so @font-face declarations are parsed
in the iframe's own document context where font-family rules can
reference them.

Also:
- Remove foliate-themes.css entirely (no longer needed)
- Set viewport background color directly via JS using THEME_COLORS map
- Remove reading theme CSS classes from viewport element

templates/reader.templ

@@ -11,7 +11,6 @@ templ Reader(user User, metadata ReaderMetadata, progress ReadingProgress, bookm
 			<title>{ metadata.Title } - Bookhoard Reader</title>
 			<link rel="manifest" href="/static/manifest.json"/>
 			<link href="/static/reader-fonts.css" rel="stylesheet"/>
-			<link href="/static/foliate-themes.css" rel="stylesheet"/>
 			<script src="/static/htmx.min.js"></script>
 			<script type="module" src="/static/reader.js"></script>
 			<link href="/static/style.css" rel="stylesheet"/>