feat(themes): free-form community themes with a security floor (#1015)

* feat(themes): free-form community themes with a security floor

Community themes are no longer token-only. The in-app guard
(validateThemeCss) now enforces only a security floor — no network
(@import / non-data url()), no scripts (<style>/<script>/expression()/
javascript:/-moz-binding), no @property, @keyframes namespaced as <id>-,
and a 256 KB cap — and otherwise allows any selectors, structure and
animations. validateThemePackage checks the manifest plus that floor.

Themes can react to app state via same-element attributes set on the theme
root: data-playing, data-fullscreen, data-sidebar-collapsed, data-lyrics-open.

The local-import confirm dialog now notes that imported themes aren't
reviewed and are installed at the user's own risk. Removes the now-unused
bundled token whitelist.

* docs(themes): note free-form themes in the Theme Store entry

Add PR #1015 and a free-form bullet to the still-unreleased Theme Store
changelog and credits entry.

src/locales/en/settings.ts

@@ -432,6 +432,7 @@ export const settings = {
   themeImportErrorDetails: 'Technical details',
   themeImportConfirmTitle: 'Install theme?',
   themeImportConfirmBody: 'Install "{{name}}" by {{author}}?',
+  themeImportConfirmRisk: "Imported themes aren't reviewed — only install themes you trust.",
   tabLibrary: 'Library',
   tabServers: 'Servers',
   tabLyrics: 'Lyrics',