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/nl/settings.ts

@@ -350,6 +350,16 @@ export const settings = {
   themeStorePagePrev: 'Vorige pagina',
   themeStorePageNext: 'Volgende pagina',
   themeStorePageStatus: 'Pagina {{page}} van {{total}}',
+  themeImportTitle: 'Een thema importeren',
+  themeImportButton: 'Thema importeren…',
+  themeImporting: 'Importeren…',
+  themeImportHint: 'Laadt een thema verpakt als .zip (manifest.json + theme.css).',
+  themeImportSuccess: '"{{name}}" geïmporteerd.',
+  themeImportErrorTitle: 'Dit thema kon niet worden geïmporteerd',
+  themeImportErrorBody: 'Het komt niet overeen met het Psysonic-themaformaat. Mogelijk is het voor een andere versie gemaakt of is het bestand onvolledig — controleer of je de juiste .zip hebt gekozen, of vraag de maker van het thema.',
+  themeImportErrorDetails: 'Technische details',
+  themeImportConfirmTitle: 'Thema installeren?',
+  themeImportConfirmBody: '"{{name}}" van {{author}} installeren?',
   tabLibrary: 'Bibliotheek',
   tabServers: 'Servers',
   tabLyrics: 'Songteksten',