v0.3.0 - React panel and download modes

Highlights

• Ship the React 18 + Vite web panel as the default UI with legacy fallback.
• Add browser-side Proxy Download, smart verified VPK upload, job rerun/delete controls, and grouped multi-part Workshop installs.
• Harden download verification with retries, resumable temp files, size checks, and VPK structure validation.
• Update README documentation with current status and follow-up TODOs.

Verification

• Built React production assets with npm run build.
• Ran Python py_compile on app.py and VPK helper scripts using local Python 3.9.

Scope note

• Ongoing local server plugin work is intentionally not included in this release.