Send runner and runCapabilities in metadata #282

Part of #277

frames.metadata() emits no runner field and no
options.runCapabilities. The app falls back to DEFAULT_CAPABILITIES, which is
all true, so Run, Rerun and Run-all render enabled for a Python run; the backend,
seeing no rerunCommand and no nightwatch marker, would take the wdio-binary
spawn path. The missing runner also leaves the locator dialect and the a11y
XPath hint on their fallbacks.

Acceptance criteria

metadata carries runner: 'selenium-webdriver'.
options.runCapabilities disables the controls the adapter cannot service, so the
UI states the refusal instead of failing on click. The locator dialect resolves to
the Selenium branch.