feat(themes): Built-in Solarized and One light/dark themes (#85)

Adds Solarized Light, Solarized Dark, One Light and One Dark as built-in themes across the workbench, editor, terminal and Mermaid, keeping the original palettes (MIT notices in THIRD_PARTY_LICENSES.md, mapping in docs/themes.md).

- Native window colours are read from tokens.css at build time (`theme_frame.rs`) instead of a second colour table.
- Startup restores the built-in palette before first paint.
- Editor schemes, the terminal and Mermaid refresh on switches between themes of the same kind.
- The plugin packages of the same themes are removed.
- A test requires every built-in palette theme to define each token (only listed ones may inherit from the base theme), so new tokens can't silently miss them.

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

docs/plugins/themes.md

@@ -1,7 +1,8 @@
 # Theme plugins
 
-OpenIDEA ships three built-in themes (Islands Light, Islands Dark, High
-Contrast). Everything else comes from **theme plugins**, the same model as
+OpenIDEA ships seven [built-in themes](../themes.md): Islands Light, Islands Dark,
+High Contrast, Solarized Light, Solarized Dark, One Light and One Dark. Additional
+themes come from **theme plugins**, the same model as
 IntelliJ IDEA, where most themes are plugins rather than part of the IDE.
 
 A theme plugin is data only. Like an IntelliJ theme (a `.theme.json` plus an
@@ -16,31 +17,6 @@ my-theme/
    └─ my-theme.scheme.json        # editor colours, optional (IntelliJ: .icls)
 ```
 
-## Ready-made theme plugins
-
-The repository's [`plugins/`](../../plugins) folder holds two complete theme
-plugins, each with a light and a dark theme and an editor colour scheme
-(syntax, editor and terminal ANSI colours):
-
-| Package | Themes | Palette |
-|---|---|---|
-| [`plugins/theme-solarized`](../../plugins/theme-solarized) | Solarized Light, Solarized Dark | [Solarized](https://github.com/altercation/solarized) by Ethan Schoonover |
-| [`plugins/theme-one`](../../plugins/theme-one) | One Light, One Dark | Atom [One Light](https://github.com/atom/one-light-syntax) / [One Dark](https://github.com/atom/one-dark-syntax) by GitHub |
-
-Both palettes are MIT-licensed; each package carries the notice in its
-`LICENSE`, which is installed with it. To use one:
-
-1. *Settings › Plugins › ⚙ › Install Plugin from Disk…* and pick the package
-   folder (`plugins/theme-solarized` or `plugins/theme-one` of a checkout or a
-   downloaded copy). Development builds also list both on the *Available*
-   tab, where *Install* does the same.
-2. Select the plugin and press *Apply* next to a theme.
-
-The IDE remembers the theme: the next start-up opens its windows, splash and
-workbench in the theme's colours before the plugin host has loaded.
-
-## Writing a theme plugin
-
 The quickest way to start is to copy [`templates/theme-plugin`](../../templates/theme-plugin).
 It contains *Harbor Night*, a complete example dark theme that sets every
 themeable token. The rest of this page covers the file format, the token
@@ -307,9 +283,8 @@ A `fontStyle: "italic"` on comments maps to `"--syn-comment-style": "italic"`.
 - Text should keep at least WCAG AA contrast (4.5:1) on its surfaces. The
   built-in themes are checked by `src/features/settings/__tests__/contrast.test.ts`.
 - Repository checks (for changes to this repo): `pnpm vitest run scripts/theme`
-  validates the template and the theme plugins in `plugins/` against
-  tokens.css and keeps this reference current, and `cargo test -p ide-plugin`
-  loads them through the plugin host.
+  validates the template against tokens.css and keeps this reference current,
+  and `cargo test -p ide-plugin` loads the template through the plugin host.
 
 ## Token reference