Docs: position VeyraKey as extension-first

[Iamsujithd]

What changed

• make the Chrome and Firefox extension the explicit primary product
• document the complete in-page login and signup workflow
• identify the toolbar popup as the full vault application
• move extension build and installation ahead of the web companion in Quick Start
• describe the web surface only as an optional development and recovery companion

Verification

• git diff --check
• CI=true pnpm lint