docs: rewrite README with a fresh structure and voice

[gnacho]

Closes #200.

Full rewrite of README.md and README.es.md, moving away from the structure inherited from the old Python codebase and its upstream README:

• New header: landing tagline, icon back, lean honest badge set (CI, version, license)
• Hero screenshot via with prefers-color-scheme, reusing the sanitized landing shots (EN shots in README.md, ES shots in README.es.md)
• New sections: What it is, first-person Why it exists (NextSync as the Rust rewrite of nextsync-py; the fork lineage lives in the nextsync-py README), What it does (grouped by theme), Providers, Install (Arch package + from source), First run, Deletions travel both ways, Files on disk, Development
• Drops the cloned mermaid flowchart, the near-verbatim Highlights bullets and the stale Project status text (the GTK UI shipped many releases ago)
• Kept: the bidirectional warning (rewritten in plain words) and the trademark disclaimer
• EN-ES parity: 9/9 sections, no em dashes, no placeholders, no fork/ehstbr references, all referenced assets exist in the repo

Rendered preview: see the Files changed tab and the rendered README in the PR.