feat(themes): import a theme from a local .zip (#1012)

* feat(themes): validate and extract locally imported theme packages

Add the backend + validation half of local theme import: users will be
able to load a theme packaged as a .zip (manifest.json + theme.css).

- `import_theme_zip` Tauri command unpacks only manifest.json + theme.css
  from the archive, outside the webview, with an archive-size cap,
  per-entry uncompressed caps, and path-traversal rejection.
- `validateThemePackage` runs the full theme-store contract in-app: the
  manifest schema (field patterns copied from the repo schema), the CSS
  token whitelist with all core tokens required, color-scheme matching
  the declared mode, data-URI restricted to the arrow token, and a guard
  against ids that collide with built-in themes. The existing
  `validateThemeCss` containment guard is reused and runs again at
  injection time. The contract token list is a byte-identical copy of the
  themes repo's allowed-tokens.json.

Covered by validateThemePackage tests for every rejection class.

* feat(themes): add the Import a theme section to the Themes tab

A dedicated section between the theme scheduler and the Theme Store lets
users import a theme from a local .zip. After the package validates, a
confirmation dialog names the theme and its author before it is installed.

A rejected import shows a plain-language explanation aimed at end users;
the raw contract diagnostics (token names, missing fields) are tucked
into a collapsible "Technical details" block for theme authors. Fully
localised across all nine languages.

* docs(themes): changelog + credits for local theme import

Fold the local .zip import (and the store pagination / refresh-scroll
polish) into the still-unreleased Theme Store entry rather than a new
section, and extend the credits line. PRs #1011 and #1012.

src/locales/ru/settings.ts

@@ -432,6 +432,16 @@ export const settings = {
   themeStorePagePrev: 'Предыдущая страница',
   themeStorePageNext: 'Следующая страница',
   themeStorePageStatus: 'Страница {{page}} из {{total}}',
+  themeImportTitle: 'Импорт темы',
+  themeImportButton: 'Импортировать тему…',
+  themeImporting: 'Импорт…',
+  themeImportHint: 'Загружает тему, упакованную в .zip (manifest.json + theme.css).',
+  themeImportSuccess: '«{{name}}» импортирована.',
+  themeImportErrorTitle: 'Не удалось импортировать эту тему',
+  themeImportErrorBody: 'Он не соответствует формату тем Psysonic. Возможно, он создан для другой версии или файл неполный — проверьте, что выбрали правильный .zip, или обратитесь к автору темы.',
+  themeImportErrorDetails: 'Технические подробности',
+  themeImportConfirmTitle: 'Установить тему?',
+  themeImportConfirmBody: 'Установить «{{name}}» от {{author}}?',
   tabLibrary: 'Библиотека',
   tabServers: 'Серверы',
   tabLyrics: 'Тексты песен',