[Phase 2 / Feature B / B-10] Docs: README passkey authenticator section + supported browsers matrix

[gynet]

Parent: #9. Depends on #52 (B-8) passing in CI. Owner: TL.

Scope

• Top-level README section: "Use NeoKeeWeb as a passkey authenticator"
• Supported browsers table (Chrome/Edge/Firefox/Safari) from design §3.7
• Security model writeup linking to `docs/phase2-passkey-design.md` §3.8
• User-facing setup guide (opt-in via options page, picking default group)
• Known limitations (SignCount across device clones, Safari unsupported)

Acceptance

• README updated
• `CLAUDE.md` Phase 2 checklist updated (Feature B shipped line)
• Links not broken (`bun run lint:docs` if configured, else manual review)