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]>

docs/plugins/themes.md

@@ -132,9 +132,29 @@ and credit the original authors in `authors` and `description`.
 
 ### From IntelliJ IDEA (automatic)
 
+To use a theme you already have in IntelliJ, open **Settings › Plugins**, gear
+menu:
+
+- **Import Theme from IntelliJ IDE…** lists the theme plugins installed in the
+  JetBrains IDEs (and Android Studio) on this computer, the theme each IDE uses
+  first. It reads their settings folders: `~/Library/Application Support/JetBrains`
+  (macOS), `%APPDATA%\JetBrains` (Windows), `~/.config/JetBrains` and
+  `~/.local/share/JetBrains` (Linux).
+- **Import IntelliJ Theme from File…** takes a theme plugin downloaded from the
+  JetBrains Marketplace (`.jar`, or a `.zip` with the plugin's `lib/*.jar`) or a
+  `.theme.json` (its `editorScheme` file is read from the same folder).
+
+The whole plugin becomes one OpenIDEA theme plugin, `intellij.<plugin id>`, with
+each of its themes and their editor schemes; it is installed through the usual
+install page and the chosen theme (otherwise the first) is applied. Importing
+the plugin again replaces it. Both are actions too (`plugins.importIntellijTheme`,
+`plugins.importInstalledIntellijTheme`). The conversion is the one described
+below.
+
 `scripts/theme/import-intellij-theme.ts` converts an IntelliJ `.theme.json`,
-and optionally its `.icls` editor scheme, into a ready-to-install plugin. It
-has no dependencies beyond Node 22.
+and optionally its `.icls` editor scheme, into a plugin folder you can edit
+before installing it — the way to port a theme for publishing. It has no
+dependencies beyond Node 22.
 
 ```sh
 pnpm theme:import-intellij path/to/MyTheme.theme.json path/to/MyTheme.icls --out ~/my-theme --author "Original Author"
@@ -179,7 +199,7 @@ ship no `.icls` at all keep the built-in syntax colours.
 | `--bg-toggle-*` | `ToggleButton.onBackground`, `.offBackground`, `.buttonColor` |
 | `--bg-feedback-*`, `--border-feedback-*` | `Banner.infoBackground`, `Banner.errorBorderColor`, … |
 
-The full table is `UI_KEYS` in [`scripts/theme/intellij.ts`](../../scripts/theme/intellij.ts).
+The full table is `UI_KEYS` in [`src/features/plugins/intellijTheme.ts`](../../src/features/plugins/intellijTheme.ts).
 
 #### `.icls` → editor scheme