theme: terminal and editor follow the app's color scheme, re-theming in place

The app resolves light/dark from prefers-color-scheme and the user's
choice (body[data-ds-dark-theme]), but xterm and CodeMirror were pinned to
hardcoded dark themes — a light-mode user got a dark card inside a light
panel. New theme.ts reads the resolved scheme + token values and notifies
on flips (the presenter toggles the body attribute); the terminal now takes
its surface colors from the tokens and a curated one-dark/one-light ANSI
palette, and the editor gets a token-driven surface with the same curated
syntax palettes. Both re-theme in place on a scheme flip: the terminal
re-applies its theme, the editor reconfigures a Compartment holding only
the scheme-dependent extensions, so the document, undo history and scroll
survive. one-dark is dropped (superseded by the curated highlight style).

src/client/theme.ts

@@ -0,0 +1,44 @@
+/**
+ * Live theme access for surfaces that cannot consume the token colors
+ * directly — xterm's palette and CodeMirror's theme extensions need concrete
+ * values, but the app's scheme flips at runtime (ui-layout's ThemePresenter
+ * projects prefers-color-scheme and the user's choice onto
+ * body[data-ds-dark-theme] and html { color-scheme }). This module reads the
+ * resolved scheme and token values, and notifies subscribers on flips, so
+ * the terminal and the editor re-theme in place instead of freezing in the
+ * scheme they happened to be created under.
+ */
+
+/** Whether the app shell resolved to the dark scheme.
+ *
+ * The presenter sets `html { color-scheme }` together with the body palette
+ * attribute, so a set color-scheme means the decision is authoritative (an
+ * absent attribute is then LIGHT even when the OS prefers dark — the user
+ * chose light). Before the presenter has run, fall back to the OS media
+ * query as the best guess.
+ */
+export function isDarkScheme(): boolean {
+  if (typeof document === 'undefined') return true
+  const decided = document.documentElement.style.colorScheme !== ''
+  if (decided) return document.body.hasAttribute('data-ds-dark-theme')
+  return typeof matchMedia !== 'undefined' && matchMedia('(prefers-color-scheme: dark)').matches
+}
+
+/** One token's computed value on <body> ('' while the theme has not applied). */
+export function tokenValue(name: string): string {
+  if (typeof document === 'undefined') return ''
+  return getComputedStyle(document.body).getPropertyValue(name).trim()
+}
+
+/**
+ * Subscribe to color-scheme flips (the presenter toggles the body
+ * attribute). The callback fires after the attribute changed; re-read the
+ * scheme inside it.
+ * @returns the disposer.
+ */
+export function subscribeColorScheme(callback: () => void): () => void {
+  if (typeof document === 'undefined') return () => {}
+  const observer = new MutationObserver(() => { callback() })
+  observer.observe(document.body, { attributes: true, attributeFilter: ['data-ds-dark-theme'] })
+  return () => { observer.disconnect() }
+}