feat(viewer): make the sidebar wordmark a home link

[benvinegar]

What

The sidebar wordmark is now a home link. Clicking "sideshow" (in the aside, or the mobile topbar) clears the current session and returns to the session-less base route — a guaranteed way back to the board from anywhere. It's a real <button>, so it's keyboard- and screen-reader-reachable.

Why

On an empty board, the way you leave a host-projected full-pane view is by clicking a session in the sidebar — but with zero sessions there are no rows to click, so there was no in-app exit (only browser-back). sideshow cloud's full-page Settings (rendered through the ss:main slot from #119) hits exactly this: open Settings on a fresh, session-less account and the only way out was the back button.

The new goHome() always asks the host to navigate — it never short-circuits on the engine's own selection — so an embedding host that layers its own view over the board gets a reliable exit through the same click; the host dedupes a no-op move. applyRoute ignores a null sessionId, so back/forward to home doesn't thrash a session load, and we deselect in goHome itself.

Scope / parity

• Pure engine-internal enhancement — no Host contract change, no new slot, nothing for embedders to adopt.
• Self-hosted behaviour is otherwise unchanged: the wordmark looks identical and now also navigates home (a standard, expected affordance).
• The mobile topbar wordmark stays content-width; only the aside wordmark fills the sidebar as a full-row tap target.

Tests

• New e2e (url-routing.spec.ts): select a session → click the wordmark → URL drops /session/:id and the row deselects.
• Full suite green locally: 205 unit tests, viewer + embed e2e (incl. the phone-width drawer test that exercises the topbar wordmark).

🤖 Generated with Claude Code