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/workbench/theme.test.ts

@@ -0,0 +1,44 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { rememberStartupTheme, STARTUP_THEME_KEY, windowBackgroundColor, windowBackgroundHex } from "./theme";
+
+const mockFrame = (token: string, resolved = "") =>
+  vi
+    .spyOn(globalThis, "getComputedStyle")
+    .mockReturnValue({ getPropertyValue: () => token, color: resolved } as unknown as CSSStyleDeclaration);
+
+afterEach(() => {
+  vi.restoreAllMocks();
+  localStorage.clear();
+});
+
+describe("window background", () => {
+  it("resolves the frame to Tauri's RGBA tuple instead of passing CSS hsl() or mixes", () => {
+    mockFrame("#26282c");
+    expect(windowBackgroundColor()).toEqual([38, 40, 44, 255]);
+    mockFrame("hsl(230, 1%, 92%)", "rgb(234, 234, 235)");
+    expect(windowBackgroundColor()).toEqual([234, 234, 235, 255]);
+    expect(windowBackgroundHex()).toBe("#eaeaeb");
+    mockFrame("color-mix(in srgb, #8040bf 80%, transparent)", "color(srgb 0.5 0.25 0.75 / 0.8)");
+    expect(windowBackgroundColor()).toEqual([128, 64, 191, 204]);
+    expect(windowBackgroundHex()).toBe("#8040bfcc");
+    mockFrame("", "");
+    expect(windowBackgroundColor()).toBeNull();
+    expect(windowBackgroundHex()).toBeNull();
+  });
+
+  it("remembers the base and the resolved splash colours for the next window", () => {
+    const root = document.documentElement;
+    root.dataset.theme = "light";
+    const values: Record<string, string> = { "--bg-frame": "#eee8d5", "--fg-muted": "#657b83", "--accent": "#268bd2" };
+    vi.spyOn(globalThis, "getComputedStyle").mockReturnValue({
+      getPropertyValue: (n: string) => values[n] ?? "",
+    } as unknown as CSSStyleDeclaration);
+    rememberStartupTheme(root);
+    expect(JSON.parse(localStorage.getItem(STARTUP_THEME_KEY)!)).toEqual({
+      theme: "light",
+      frame: "#eee8d5",
+      fg: "#657b83",
+      accent: "#268bd2",
+    });
+  });
+});