Show the site title in the admin instead of the generic EmDash mark

[swissky]

Problem: The admin sidebar header (and footer) always reads "EmDash" unless a build-time admin.siteName is configured. Operators running several EmDash backends can't tell at a glance which site's admin they're in — every tab looks identical.\n\nProposal: WordPress-style branding: when no explicit admin.siteName is set, fall back to the site's own title in the admin manifest. Precedence: build-time admin.siteName → Site Title (Settings → General) → the title captured by the setup wizard → the bundled "EmDash" default.\n\nThis complements #1505 (Site Icon as admin favicon) — together, favicon + sidebar make each backend instantly recognizable.\n\nImplementation is a small fallback in the manifest route (no schema/API change, admin SPA untouched — it already renders manifest.admin.siteName). PR ready at the linked pull request.