[P2] Add `<h1>` + Open Graph meta tags for SEO + social previews

[rz1989s]

Summary

DOM has only one heading (`

ACTIVITY STREAM · LAST 24H`). The product name "SIPHER" is in a generic ``. No `

`. No `og:title`/`og:description`/`og:image` for Twitter card preview. No `twitter:card`/`twitter:image`.

When the X thread #1 launches and someone tweets `https://sipher.sip-protocol.org\`, the recipient sees a bare URL preview with no rich card. SEO + accessibility (screen reader hierarchy) + social-share preview all suffer simultaneously.

Source findings

• End-user QA Fresh-eyes `PU-2`

Affected files

• `app/index.html` (or whatever Vite-side template the app uses) — add meta tags
• Wherever the header brand mark "SIPHER" lives — wrap in `

  `

• Add a 1200x630 hero image to `public/` for og:image

Suggested fix

```html

<title>SIPHER — Privacy Command Center</title> \`\`\`

In the React tree, wrap the brand mark in `

` (visually styled to match current span) so screen reader hierarchy starts correctly.

Verification

• Open https://cards-dev.twitter.com/validator and paste the URL
• Run a Lighthouse SEO audit
• Use a screen reader to navigate the heading hierarchy

Cross-ref hints

og tags, twitter card, h1 missing, SEO, social preview, accessibility heading hierarchy, X thread launch readiness