feat: user dashboard, gated downloads, Google OAuth sign-in #4763

Originally written by @ccantynz-alt on GitHub.
Imported from https://github.com/ccantynz-alt/voxlen/pull/38

Summary

• User dashboard at /#dashboard — shows subscription status, download links, account info
• Admin access for ccantynz@gmail.com — full access badge, no API key required, all features unlocked
• Gated downloads — Free Trial / Download buttons require Google sign-in before the download link is revealed
• Post-login redirect — signing in anywhere on the landing page sends the user straight to the dashboard
• Navbar dropdown — signed-in users see Dashboard, Open App, Sign out

Test plan

• Click "Start Free Trial" without signing in — Google OAuth popup appears
• Sign in with ccantynz@gmail.com → dashboard shows Admin / Full Access badge with all privileges listed
• Sign in with another Google account → dashboard shows Free Plan + upgrade prompt + API key links
• Dashboard download links resolve to GitHub release assets
• Sign out → returns to landing page, download button reverts to sign-in prompt
• Refresh while on /#dashboard → stays on dashboard if still signed in

https://claude.ai/code/session_011MFNHeFKrhfuSrEYVgrdyr

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