Make the theme a prop, next to the language

The theme is state — what Element Call should look like right now — and
so belongs beside `language` as a prop, not on the imperative handle
(where it was a request, `setTheme`, because the internal host bridge
speaks the widget API and a widget's host sends theme changes as
requests) and not in the configuration (where `config.theme` only ever
set the starting theme).

The `theme` prop feeds the same channel the rest of Element Call listens
to for a host's theme, replayed so that whatever subscribes after the
host has set it still hears the current one. Changing it re-themes the
container and nothing else; unlike the language, it is per component.
`setTheme` and `config.theme` are gone, and the harness gets a theme
picker in place of its per-pane buttons.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

README.md

@@ -248,7 +248,8 @@ The component speaks every language the app does. English is bundled in; the
 other locales are split into chunks the host's bundler loads the first time
 they are needed. It starts in the browser's language, and follows the host's
 own language setting through the `language` prop (`supportedLanguages` lists
-the tags it accepts).
+the tags it accepts). The `theme` prop works the same way for `light` and
+`dark`; both can change while a call is running without disturbing it.
 
 The package is not published yet. A host installs it as a git dependency on the
 `component` directory of this repository,