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/en/settings.ts

@@ -421,6 +421,16 @@ export const settings = {
   themeStorePagePrev: 'Previous page',
   themeStorePageNext: 'Next page',
   themeStorePageStatus: 'Page {{page}} of {{total}}',
+  themeImportTitle: 'Import a theme',
+  themeImportButton: 'Import theme…',
+  themeImporting: 'Importing…',
+  themeImportHint: 'Load a theme packaged as a .zip (manifest.json + theme.css).',
+  themeImportSuccess: '"{{name}}" imported.',
+  themeImportErrorTitle: 'This theme could not be imported',
+  themeImportErrorBody: "It doesn't match the Psysonic theme format. It may have been made for a different version, or the file is incomplete — check that you picked the right .zip, or ask the theme's author.",
+  themeImportErrorDetails: 'Technical details',
+  themeImportConfirmTitle: 'Install theme?',
+  themeImportConfirmBody: 'Install "{{name}}" by {{author}}?',
   tabLibrary: 'Library',
   tabServers: 'Servers',
   tabLyrics: 'Lyrics',