fix(ui): lock light theme and reliable media playback

[ppapsmoken]

Background

The console and public site could follow the browser/system dark-mode preference, and the Overview hero copy carried a trailing colon in every locale. The Playground conversation rail also needed to be open by default on desktop only. The homepage video-model panel could remain on its poster frame when the tab became visible.

Evidence / reproduction

• Setting the browser to dark mode changed the console/classic/site appearance.
• Overview locale strings rendered a trailing :.
• Desktop and mobile shared the same collapsed initial state for the conversation rail.
• The second-screen video panel contained an MP4 but could show only its poster after tab activation.

Scope / root cause

• Remove system-theme detection and force the light class/color scheme across default, classic, and website surfaces.
• Make the conversation rail default expanded at min-width: 768px, collapsed below that breakpoint.
• Keep the Overview logo CDN path with a local PNG fallback.
• Rename the Overview i18n key/value without the trailing colon in all eight languages.
• Explicitly call play() on the muted inline video at canplay so tab activation reliably starts the MP4.

Impact / risks

UI-only changes across console and website; no API, router, billing, database, or infrastructure changes. The website workflow and console workflow will deploy independently from the same merge.

Validation

• web/default: bun run typecheck, bun run build:check
• web/classic: bun run build
• website: bun run lint, bun run typecheck, bun run build
• MP4 verified as H.264, 790x400, 5 seconds.