chore(console): remove drifting SDUI dev preview scaffolding (#2115)

scenario-sources.ts + sdui-scenarios-preview.* + sdui-source-editor-preview.*
were manual dev preview harnesses used while building the react-tier showcase
pages and the Studio source editor. They inline-COPY the real showcase page
sources and aren't wired into any build or CI, so they silently drift from the
authoritative pages in framework examples/app-showcase. The real regression
coverage now lives in the showcase-smoke e2e plus the spec↔frontend conformance
ratchet and the react-prop gate — these copies only rot.

Kept: manifest-dump.{html,tsx} (load-bearing — dump-public-manifest.mjs →
build-console.sh produces sdui.manifest.json from it) and __tests__.

Co-authored-by: Jack Zhuang <277994282+os-zhuang@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: os-zhuang