fix: keep chart in sync with toolbar on background stat refreshes

[rajbos]

Problem

The chart and toolbar were showing different "today" token counts. For example, the toolbar would show 17M tokens used today while the chart only showed ~4.3M for the same day. The details screen was also counting more sessions (20) than the chart (3).

Root cause

_runUpdateTokenStats() runs every 5 minutes and correctly refreshes lastDailyStats (fresh 30-day window) to update the toolbar and details panel. When it also pushes an update to the open chart panel, it prefers lastFullDailyStats (the 365-day dataset computed once when the chart was first opened) -- but that full-year dataset is never refreshed by the background timer. So the chart always displayed the stale snapshot from when the chart was opened, while the toolbar reflected current data.

Fix

After each calculateDetailedStats() run, merge the fresh 30-day dailyStats entries into lastFullDailyStats by upsert on the date key. Sessions older than 30 days are unaffected (their historical dates are stable), while the recent window stays accurate on every background refresh.

if (this.lastFullDailyStats) {
    const fullMap = new Map(this.lastFullDailyStats.map(d => [d.date, d]));
    for (const day of dailyStats) {
        fullMap.set(day.date, day);
    }
    this.lastFullDailyStats = Array.from(fullMap.values()).sort(...);
}

This is a surgical 9-line addition in extension.ts with no API or schema changes.