feat(themes): Solarized and One theme plugins (#92)

Solarized Light/Dark and One Light/Dark ship as theme plugins (plugins/theme-solarized, plugins/theme-one) with complete UI tokens, editor schemes and terminal colours, and the original palettes' MIT notices. Installing a plugin now also copies its LICENSE/NOTICE/COPYING.

Theme handling that benefits every plugin theme:
- The last plugin theme is restored before first paint (splash, workbench); the native window background follows the theme.
- HSL / color-mix() / var() colours are resolved to RGB for native windows, the Settings window and Mermaid.
- Mermaid, the terminal and editor schemes follow switches between two themes of the same kind.
- An explicitly chosen editor scheme wins over a plugin theme's editor colours.

Supersedes #85.

Co-authored-by: bghtyu <[email protected]>
Claude-Session: https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn

src/editor/core/colorSchemeRuntime.ts

@@ -26,7 +26,10 @@ export function installColorScheme(): () => void {
     if (s.settings !== prev.settings) apply();
   });
   const observer = typeof MutationObserver !== "undefined" ? new MutationObserver(apply) : null;
-  observer?.observe(document.documentElement, { attributes: true, attributeFilter: ["data-theme", "data-contrast"] });
+  observer?.observe(document.documentElement, {
+    attributes: true,
+    attributeFilter: ["data-theme", "data-contrast", "data-plugin-theme"],
+  });
   return () => {
     unsubscribe();
     observer?.disconnect();