feat(plugins): import IntelliJ theme plugins (#1184)

Settings › Plugins (gear menu) gets Import Theme from IntelliJ IDE… (theme plugins of the JetBrains IDEs on this machine, the one each IDE uses first) and Import IntelliJ Theme from File… (a Marketplace .jar / .zip or a .theme.json). The host reads the plugin with size limits; the UI converts it with the IntelliJ mapping (moved from scripts/theme to src/features/plugins/intellijTheme.ts, still used by pnpm theme:import-intellij) into one OpenIDEA theme plugin (engines.openidea ^1.0) with all of its themes, installed through the usual install page, and applies the chosen theme.

Theme pickers separate each plugin's themes, and Quick Switch Scheme › Theme previews the theme under the pointer or keyboard after 500 ms and restores the selected one when closed without a choice.

Co-authored-by: ldg030201 <[email protected]>

plugins/README.md

@@ -728,7 +728,9 @@ granted, and starts the approved program.
 - **Install:** Settings › Plugins › _Install Plugin from Disk…_ (a package
   folder) or _Install Plugin from Package File…_ (a `.oiplugin`, a `.zip` with
   the package at its root or in its only folder, or a folder's `plugin.toml`;
-  a zip without a `plugin.toml` there, such as an IntelliJ plugin, is refused). The install
+  a zip without a `plugin.toml` there, such as an IntelliJ plugin, is refused; IntelliJ
+  theme plugins are imported instead: _Import IntelliJ Theme from File…_ and _Import Theme
+  from IntelliJ IDE…_, see [`docs/plugins/themes.md`](../docs/plugins/themes.md)). The install
   page lists every capability and whether the package is signed by a publisher
   the marketplace trusts; nothing is copied until you accept.
 - **Versions:** installed plugins live in `<app-data>/plugins/<id>/<version>/`