Rewrite the README around what users need to know

[squarepots]

Summary

Rewrite the English and Simplified Chinese READMEs around what a user needs:

• what GSwitch does, as short bold-led points;
• install per platform, with the unsigned Windows installer, the WebView2 requirement, and what to do when its download stalls;
• adding accounts, including Cockpit Tools import: what Find on this computer reads, that it previews before importing, never replaces saved accounts, never changes Cockpit Tools' files, and leaves Cockpit-only data behind;
• what to do before switching (install Codex, quit it, file-backed sign-in);
• where credentials are stored and which services GSwitch contacts.

Removed: the OpenAI affiliation disclaimer, the screenshot note, the PowerShell helper section, and the signing-policy link. Short uninstall steps stay because docs/code-signing-policy.md points to them.

Validation

Documentation only. git diff --check passed, and every relative link and image path exists.