Themes: Dracula, Material, Adwaita, Flat Remix, Paper, Nordic, each light, dark or Auto

The theme picker lists Modern (the old Dark and Light), Classic and six new
palettes, from Dracula's spec (with Alucard), Material 3's baseline scheme,
libadwaita's CSS variables, Flat Remix's _colors.scss, Paper and Nord. A second
setting picks Light, Dark or Auto where a theme has both; Classic and Paper do
not, so it is hidden for them.

The page gets data-mode, light or dark, and Auto is worked out in theme.ts from
the system, so each palette is written once instead of again under a media
query. Every new palette clears WCAG AA for text on its backgrounds. An old
ipx.theme of dark, light or auto carries over as Modern.

Closes #27.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web/build.mjs

@@ -15,7 +15,7 @@ const here = path.dirname(fileURLToPath(import.meta.url));
 // The files are one script, concatenated in this order, not modules: they share one top-level
 // scope, as the single inline script did, and code that runs at load needs what came before it.
 const PAGES = {
-  'index.html': ['util', 'feeds', 'feedpage', 'items', 'player', 'dialogs', 'events'],
+  'index.html': ['util', 'theme', 'feeds', 'feedpage', 'items', 'player', 'dialogs', 'events'],
   'login.html': ['login'],
 };