[FE-97] Add metadata, favicon, and SEO basics

[yusuftomilola]

Overview

As an open-source project with a public landing page (app/page.tsx), AssetsUp needs proper metadata — both for discoverability and so shared links render correctly.

Tasks

• Add metadata exports with unique titles and descriptions, using a title template so pages read as "Assets — AssetsUp".
• Add Open Graph and Twitter card tags with an OG image for the public landing page.
• Add a favicon and app icons.
• Add app/robots.ts and app/sitemap.ts covering public pages only, and mark authenticated dashboard routes noindex.
• Set metadataBase so relative OG image URLs resolve correctly.
• Add a web app manifest with name, icons, and theme color.

Acceptance Criteria

• Every page has a unique, descriptive title.
• Sharing the landing page renders a correct preview card.
• Dashboard routes are excluded from indexing.

Notes for Contributors

Comment below to be assigned.