seo: root layout metadata gaps — no twitter card, no canonical, themeColor contradicts the manifest

[shauryagangrade]

Problem

src/app/layout.tsx:15-44 metadata is missing several pieces:

1. No twitter card block (X/Discord unfurls fall back inconsistently; openGraph exists)
2. No alternates.canonical — with trailing-slash/query variants this invites duplicate URLs
3. viewport.themeColor is hardcoded dark #0A0A0A even in light mode, while public/manifest.webmanifest uses amber #F59E0B — three brand colours disagreeing

Fix

Add the twitter block + canonical; use media queries for light/dark themeColor consistent with next-themes; align manifest colour or document why they differ.

Acceptance criteria

• Twitter card renders on x.com/Discord
• themeColor follows the active theme