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-tauri/src/lib.rs

@@ -5,6 +5,7 @@ pub mod cli;
 mod cover_cache;
 mod library_analysis_backfill;
 mod lib_commands;
+mod theme_import;
 
 pub use psysonic_integration::discord;
 
@@ -593,6 +594,7 @@ pub fn run() {
         })
         .invoke_handler(tauri::generate_handler![
             greet,
+            theme_import::import_theme_zip,
             backup_export_library_db,
             backup_import_library_db,
             backup_export_full,