docs: trim README to a non-technical, user-facing page

[vreshch]

Removes the developer/technical sections so the README reads for end users:

• Building from source (npm/esbuild/vitest commands) - removed.
• How it works (git internals, isomorphic-git transport, force-push semantics) - removed.
• Mobile (planned) (isDesktopOnly internals) - removed; the desktop-only status is now a one-line banner.

Also de-jargoned the leftover references (status banner, Features sign-in/merge bullets, FAQ) and fixed the now-dead #mobile-planned anchors. Kept the required disclosures (Privacy network/hosts + isomorphic-git attribution) so check:docs/check:hosts still pass. Docs-only; no version bump.

[github-actions]

🎉 PR Validation ✅ PASSED

Commit: 3335b4d59a50a7a5fb810702bbfa1b8d30a7776d
Branch: docs/readme-trim

Checks:

• ✅ Dependencies installed
• ✅ Type check passed
• ✅ Linting passed
• ✅ Format check passed
• ✅ Tests + coverage passed
• ✅ Build successful
• ✅ Store-compliance checks passed
• ✅ Bundle is mobile-safe

Ready to merge! ✨

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