Fix the version dropdown, and document icons, Analytics and the sign-in host

[seenu-k]

Four unrelated reports, all implementor-facing.

1. The version dropdown

dropdownItemsAfter pinned reference/compatibility into the version dropdown. Each version entry links to the doc you are currently reading, so on the compatibility page itself the two entries collided. From the built HTML before this change:

<a class="navbar__link active" aria-current=page href=/docs/reference/compatibility>0.3.x</a>
<a class="dropdown__link dropdown__link--active" href=/docs/reference/compatibility>0.3.x</a>
<a class="dropdown__link dropdown__link--active" href=/docs/reference/compatibility>Compatibility</a>

Identical hrefs, both active. Docusaurus' own example pins /versions, a page outside /docs that no version owns — that is the case the option is for. Compatibility is already in referenceSidebar, so nothing becomes unreachable.

dropdownActiveClassDisabled: true follows, because the entry is a self-link on every docs page and therefore permanently active, which says nothing when there is one version. After, on both compatibility and intro:

<a class="navbar__item navbar__link" href=…>0.3.x</a>

2. Icons (branding)

Separates the icon question from the web build, matching eigen-server#55: the engine serves a placeholder until a game has icons of its own, and a game with no web build now gets its own icons by copying them into public/.

3. Google Analytics

configure.md implied the console and CLI project-create flows were equivalent ("with Analytics enabled, or let step 2 do it"). Only the console links an Analytics account, so a CLI-created project reports "Google Analytics not enabled for Project" and the engine's automatic screen_view and seven events have nowhere to land. Corrected there, plus a new quickstart step naming the banner and the fix.

4. The sign-in host

New section in Deploy the web app: what the firebaseapp.com string is, why leaving it alone is fine, and the two independent changes if you want your own name there — the OAuth consent screen (no domain needed) and AUTH_DOMAIN (Firebase Hosting domain needed). Stresses that AUTH_DOMAIN is not APP_HOST. Reference tables in configure.md and manual-setup.md updated.

Merge order

This should land after the engine and scaffolder are published (eigen-server#55, eigen-server#56, eigen-flutter#34). The site is the source of truth and deploys on merge, so it should not describe an AUTH_DOMAIN or a placeholder icon nobody can install yet. The dropdown fix is independent and only waits for the same reason.

Verification

pnpm build clean with onBrokenLinks: "throw", plus check-admonitions, check-docs-version and typecheck. Dropdown markup verified in built output, not by inspection.

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