Add Apple Screen Time-inspired usage dashboard with virtual scrolling

[ejc3]

Note: This PR should be merged after #20 (request comparison feature)

Overview

This PR adds a comprehensive usage analytics dashboard inspired by Apple Screen Time, along with performance optimizations for displaying thousands of API requests.

New Features

📊 Usage Dashboard

A visual analytics interface showing Claude API usage patterns:

Weekly View

• Bar chart showing token usage by day (Sunday-Saturday)
• Color-coded by model (Purple=Opus, Blue=Sonnet, Green=Haiku)
• Hover tooltips with per-model breakdown
• Average usage line indicator
• Displays total week usage

Today by Hour

• Hourly activity distribution throughout the day
• Current time indicator (vertical line at present hour)
• Per-model breakdown in each hour
• Identify peak usage times

Model Statistics

• Horizontal bars showing proportional usage across models
• Total tokens and request counts per model
• Visual comparison of model distribution

Summary Metrics

• Total tokens with K/M formatting
• Request count
• Average response time
• Average tokens per request

⚡ Virtual Scrolling

• Handles 5000+ requests without browser freezing
• Only renders visible items + 10 overscan
• Smooth 60fps scrolling
• Uses TanStack Virtual v3.13.12

🗓️ Smart Date Navigation

• Previous/Next day buttons
• Shows "Today" when viewing current date
• Shows date (e.g., "Nov 29") for other dates
• Consistent labeling in selector and dashboard

🔧 New API Endpoints

• GET /api/stats/hourly?start={UTC}&end={UTC} - Hourly breakdown
• GET /api/stats/models?start={UTC}&end={UTC} - Model statistics
• GET /api/requests/summary?start={UTC}&end={UTC} - Lightweight summaries
• GET /api/requests/{id} - Single request details
• GET /api/stats?start={UTC}&end={UTC} - Enhanced with per-model breakdowns

🌍 Timezone Handling

• Browser calculates local day boundaries → UTC timestamps
• Backend timezone-agnostic (receives exact UTC ranges)
• SQLite uses datetime() for proper comparisons
• "Today" always correct regardless of timezone

🗄️ Database Improvements

• WAL mode for concurrent access
• 5000ms busy timeout prevents lock errors
• Per-model statistics in aggregates
• Cache tokens included in calculations

Performance

Before: 5000 requests → 30s browser freeze
After: Instant render, 60fps scrolling

Stats loading: 3 parallel requests in ~200ms
Week navigation: 3x faster (only reloads when changing weeks)

Implementation

Frontend:

• UsageDashboard.tsx - Dashboard component
• UsageDashboard.css - Custom styling
• Virtual scrolling with useVirtualizer
• getLocalDayBoundaries() for timezone calc
• Parallel stats loading

Backend:

• GetHourlyStats() - Hourly breakdown with models
• GetModelStats() - Model aggregates
• Enhanced GetStats() with per-model data
• All queries use datetime() for timezone awareness

Testing

✅ 5000+ requests
✅ Multiple timezones (PST/EST/UTC)
✅ Week navigation
✅ Model filtering
✅ Concurrent DB access

Breaking Changes

None - fully backward compatible