docs: position GoBarryGo as a native work suite

[shreyam1008]

What changed

• Reposition GoBarryGo as a low-overhead native general-work suite with Downloader / Inbox as the only shipped module.
• Add the staged product roadmap for downloader trust, File Workspace, Batch Tools, and later Workflows.
• Align README, product manifest, architecture, native UI intent, changelog, domain/distribution records, and site SEO/copy.
• Add a public roadmap section without claiming future modules are implemented.

Validation

• site: bun run verify
• frontend: production build and bundle budget pass
• Chrome rendered check: custom local build loads with no warnings/errors; roadmap navigation reaches the new section.
• Existing frontend test suite has one pre-existing window is not defined failure in the Wails runtime store test; Go GUI tests require unavailable GTK/WebKit system packages.