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/de/settings.ts

@@ -354,6 +354,16 @@ export const settings = {
   themeStorePagePrev: 'Vorherige Seite',
   themeStorePageNext: 'Nächste Seite',
   themeStorePageStatus: 'Seite {{page}} von {{total}}',
+  themeImportTitle: 'Theme importieren',
+  themeImportButton: 'Theme importieren…',
+  themeImporting: 'Importiere…',
+  themeImportHint: 'Lädt ein als .zip verpacktes Theme (manifest.json + theme.css).',
+  themeImportSuccess: '„{{name}}“ importiert.',
+  themeImportErrorTitle: 'Dieses Theme konnte nicht importiert werden',
+  themeImportErrorBody: 'Es entspricht nicht dem Psysonic-Theme-Format. Vielleicht wurde es für eine andere Version erstellt oder die Datei ist unvollständig — prüfe, ob du die richtige .zip gewählt hast, oder frag den Autor des Themes.',
+  themeImportErrorDetails: 'Technische Details',
+  themeImportConfirmTitle: 'Theme installieren?',
+  themeImportConfirmBody: '„{{name}}“ von {{author}} installieren?',
   tabLibrary: 'Bibliothek',
   tabServers: 'Server',
   tabLyrics: 'Songtexte',