Add chart panel for day-by-day token usage visualization

[Copilot]

Adds an interactive chart view showing GitHub Copilot token usage aggregated by day, accessible from the details panel.

Changes

Data aggregation

• DailyTokenStats interface for daily metrics (date, tokens, sessions, interactions)
• calculateDailyStats() aggregates session files by modification date using Map-based grouping
• formatDateKey() helper for consistent YYYY-MM-DD formatting
• Reuses existing session file cache for performance

Chart panel

• showChart() creates webview with Chart.js visualization
• Bar chart for daily tokens (left Y-axis) + line overlay for sessions (right Y-axis)
• Summary cards: total days, total tokens, avg/day, total sessions
• Refresh handler to reload data on demand

UI integration

• "Show Chart" button added to details panel footer
• New command copilot-token-tracker.showChart registered
• Panel opens in separate column via ViewColumn.Beside
• Webview message routing for 'showChart' and 'refresh' commands

Chart rendering

private getChartHtml(dailyStats: DailyTokenStats[]): string {
  const labels = dailyStats.map(stat => stat.date);
  const tokensData = dailyStats.map(stat => stat.tokens);
  const sessionsData = dailyStats.map(stat => stat.sessions);
  
  // Pre-calculate to avoid duplicate reduce() calls
  const totalTokens = dailyStats.reduce((sum, stat) => sum + stat.tokens, 0);
  // ... generates HTML with Chart.js from CDN
}

Chart.js loaded via CDN (no new npm dependencies). Current month data only to keep scope manageable.

Original prompt

Add a chart panel that can be triggered from the report overview. the new panel shows the toke usage over time, on a day by day basis.

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