Add splash-screen smart tag; centralize site title/tagline defaults

#29 in Riparion/riparion-cms — merged 2026-06-01

Summary

• Add a splash smart tag to the embed registry — a full-bleed hero (title, subtitle, optional CTA) reachable from the page editor's + Smart tag menu, with optional border / background. Title and subtitle default to the live site branding; href defaults to /blog.
• Centralize the site title/tagline defaults so the literals live in exactly one place: private consts in settings.rs, applied only by site_title_or_default / site_tagline_or_default. get_site_* and the feed generator funnel through them; every consumer (header, footer, layout headings, splash, head tags, feed) now reads the live, default-applied value and never references a default constant.
• Resolve the shared SiteChrome context at SSR via a new SiteChromeProvider (use_server_future), so the branding is in the server HTML — no empty/flashing brand before hydration, correct for crawlers, and no per-consumer fallbacks needed.

New install initial values: Riparion CMS / A Dioxus-based Content Management System (returned by the getters when unset — no seeding involved).

Test plan

• cargo check --features server
• cargo check wasm/web target (--no-default-features --features web,sqlite --target wasm32-unknown-unknown)
• cargo fmt
• cargo clippy on both targets (no new warnings)
• Manual: insert + Smart tag → Splash screen in the page editor; confirm it renders the site title/tagline and CTA, and that border=false / background=false strip the frame/wash.
• Manual: change the site title/tagline in Settings; confirm header, footer, layout headings, and splash all track it after reload.

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