feat(viewer): full-page standalone view for surface direct links

[benvinegar]

What & why

Visiting a bare /s/:id direct link now opens a full-page standalone view of that one surface — title and parts only, no sidebar, session feed, or comment thread — with a small "made with sideshow" watermark beneath it.

/s/:id used to render a standalone surface page. Two refactors left it resolving into the session feed instead:

• feat(viewer): render rich parts server-side, served from /s/:id #125 made surfaces multi-part, each part its own sandboxed iframe at /s/:id?part=N, dropping the whole-surface page.
• feat: link unfurl / inline preview support (Open Graph + Twitter Cards) #130 repurposed bare /s/:id to serve the viewer SPA (for link-preview metadata), which then resolved the link into its session and dropped you in the feed.

This restores the standalone view within the current architecture.

How

• state.ts — a standaloneSurface signal + bootstrap() entry point. A bare surface route (surfaceId, no session) fetches the surface and enters standalone mode; applyRoute toggles it on back/forward. Falls back to the board if the surface can't be fetched.
• App.tsx — renders StandaloneView (the one surface + watermark) instead of the board chrome, and titles the tab after the surface.
• Card.tsx — a standalone prop strips the version dropdown, "updated" meta, comment thread, and scroll/URL observer — keeping the iframe registration so parts render in the same sandboxed frames sized by the same resize bridge.
• styles.css — centered single-surface layout + watermark.

No server change: bare /s/:id still serves the SPA shell with link-preview metadata (#130); the viewer picks the layout from the route. Works under the deployed /u/:user prefix via appPath/basePath.

Updated the url-routing e2e test (it pinned the old feed behavior) and added a changeset (minor).

Notes

• The standalone view is a snapshot — it doesn't live-update if the surface is revised while open (a reload shows the latest). Easy to wire up later if wanted.

Validation

• typecheck, lint, format:check, full build — all clean.
• Unit suite: 232 pass.
• e2e: chromium passed all url-routing / embed-stream / public-read specs including the new standalone test (asserts #standalone present, no #sessionList, no comment thread, watermark href, URL stays /s/:id, authored HTML renders only inside the sandboxed iframe). Webkit couldn't launch in the local sandbox (missing system libs); runs in the separate CI job.

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