feat(website): brand layer — teal palette, table logo, wordmark, footer

Anchor the site to the product's identity (Phase B branding):
- Accent palette mapped to the app's in-TUI teal (--sl-color-accent-*,
  light + dark); trim the splash hero's oversized desktop bottom padding.
- Header logo: a database-table glyph with a teal primary-key cell
  (light/dark variants); favicon redrawn to match.
- Landing wordmark 'RELational PLAYground': teal picks out REL+PLAY
  ('relplay', the domain) and R/D/B/M/S (spelling out RDBMS). Sizes
  locked in em off one master scale so the lockup zooms as a rigid unit.
- Footer override: default footer + an understated company (Lazy
  Evaluation Ltd) and source/issues line.

No engine names or 'DSL' in user-facing copy.

website/src/content/docs/index.mdx

@@ -15,6 +15,9 @@ hero:
 
 import { Card, CardGrid, LinkCard } from '@astrojs/starlight/components';
 import Demo from '../../components/Demo.astro';
+import Wordmark from '../../components/Wordmark.astro';
+
+<Wordmark />
 
 RDBMS Playground is a cross-platform terminal app that gives you a safe
 sandbox for exploring relational database concepts: tables, columns,