feat: Next.js dashboard — subscription list with create/pause/delete

[cokehill]

Summary

The dashboard is the face of stellar-notify for non-API users. It must provide a clean UI
for full subscription management.

Acceptance criteria

• API-key entry screen with sessionStorage persistence
• Subscription list shows: type badge, channel, label, active status, account filter
• "New Subscription" modal with fields for all types and channels
• Toggle active / paused without page reload (optimistic update)
• Delete with confirmation dialog
• Stats bar: active count, total, delivered, pending, failed
• Mobile-responsive layout
• Dark space theme using CSS custom properties

Design notes

Font: IBM Plex Mono + DM Sans.
Color accent: #2ab9b9.
Background: #040b14 with subtle star-field pseudo-element.