docs: Update README with new features and stats #80

Summary

The README needs updating to reflect recent additions and current project stats.

New Features to Add

v0.0.76+

• HTTP Range Requests - Video/audio seeking support (206 Partial Content) - feature: add HTTP Range request support for video/audio seeking #68, PR feat: add HTTP Range request support for media streaming #69
• Passkey Authentication - WebAuthn/FIDO2 passwordless login - feat: Add passkey (WebAuthn) authentication support #78, PR feat: add passkey (WebAuthn) authentication support #79
• SolidOS UI Mode - --solidos-ui flag for modern Nextcloud-style interface
• Single-User Mode - --single-user for personal pod servers

Stats to Update

Current codebase:

• Source code: ~13,000 lines (was likely less before)
• Test code: ~4,700 lines
• Total: ~17,700 lines
• Test count: 223+ tests
• Dependencies: 18 direct dependencies

Breakdown by module:

Version Update

Change "Implemented (v0.0.75)" to current version.

Feature List Updates

Add to features list:

- **HTTP Range Requests** - Video/audio seeking, partial content delivery (206)
- **Passkey Authentication** - WebAuthn/FIDO2 passwordless login with Touch ID/Face ID
- **SolidOS UI** - Modern file manager interface (`--solidos-ui`)
- **Single-User Mode** - Simplified setup for personal pods (`--single-user`)

CLI Options to Document

New flags that may be missing:

• --solidos-ui - Enable SolidOS modern UI
• --single-user <webid> - Single-user mode for personal servers
• --range-requests (if configurable, or just note it's always on)

Optional: Add Stats Section

Consider adding a "Project Stats" section:

## Project Stats

- ~13,000 lines of source code
- ~4,700 lines of tests (223+ tests)
- 18 direct dependencies
- Zero native dependencies (pure JavaScript, runs on Termux/Android)