2d: Release Health dashboard tab — crash rate + session breakdown

[ajianaz]

Parent: #232 · Blocked by: #241, #242

Scope

Add "Release Health" tab to project dashboard.

UI Components

Release Health Tab

• Crash rate card: big number display, trend arrow (↑/↓), per release
• Session breakdown bar: stacked horizontal bar — exited (green) / errored (yellow) / abnormal (orange) / crashed (red)
• Release selector: dropdown to filter by release version
• Environment toggle: tabs for production vs development
• Timeline chart: crash rate over time (line chart, configurable window)
• Per-release table: release, env, crash rate %, total sessions, error count, last seen

Crash Rate Calculation

crash_rate = crashed / (exited + errored + abnormal + crashed) × 100

API Endpoints

• GET /api/projects/:id/release-health?release=x&env=y&page=1
• GET /api/projects/:id/release-health/summary — per-release overview
• GET /api/projects/:id/release-health/crash-rate?release=x&env=y&window=24h

Tasks

• Create +page.svelte for release health tab
• Crash rate card component with trend indicator
• Session breakdown bar chart
• Release version selector
• Environment filter (prod/dev)
• Timeline chart (crash rate over time)
• Empty state: "No session data yet — ensure Sentry SDK has release configured"
• Responsive layout

Effort: ~3 hours