Branded home button in the sidebar for self-hosted instances

Use case

Self-hosted instances typically embed Fluxer as the community layer of a larger product (in our case: a language school whose website handles accounts, billing and scheduling, with Fluxer as the campus/community environment). Users arrive from the operator's website; they need an obvious way back. Today the top-left sidebar button (FluxerButton) hardcodes the Fluxer symbol and always navigates to /channels/@me. The pieces for a branded home button already exist — instance branding symbol_url and the marketing endpoint are both delivered to RuntimeConfig — the sidebar just doesn't use them.

Proposal

When the instance's marketing endpoint points at a different origin than the web app (true for self-hosted instances that set FLUXER_MARKETING_ENDPOINT to their product site; false on hosted fluxer.app):

1. Render a branded instance home button (the instance symbol_url) at the very top of the sidebar; clicking it navigates to the marketing endpoint.
2. The existing Direct Messages button renders directly beneath it, unchanged.

Hosted fluxer.app keeps today's sidebar untouched (same-origin check fails ⇒ button hidden).

Offer

We have this working as a small additive patch (one new component + a 2-line GuildsLayout insert, no modified upstream logic) and are happy to submit it as a PR if the direction is acceptable — including any adjustments (config flag instead of origin-heuristic, admin toggle, etc.). Related context: we run the closed self-hosted instance previously discussed in #707/#1292/#1293.