Fix trailing 7-day stats and add 14-day totals legend

[NickPPC]

Summary

• Fix the dashboard chart so the "Last 7 days" data is a true rolling 7-day window (orders before now - 7 days are no longer counted, and days with zero orders are no longer silently dropped).
• Build the chart's day axis deterministically from today's local midnight back through today - 7 days, padding missing days with empty buckets.
• Add a "Last 14 days" legend rendered side-by-side with the existing "Last 7 days" legend, backed by a new loadRobotTotalsLastNDays helper.

Test plan

• Load the dashboard; bar chart shows day groups from today - 7 days through today, with empty groups rendered for days with no orders.
• "Last 7 days" legend tallies only orders inside the rolling 7×24h window (loaded at 11am Monday, an order from last Monday 9:30am is excluded).
• "Last 14 days" legend appears alongside the 7-day one and shows totals ≥ the 7-day numbers; may include robots that don't appear in the 7-day legend.
• Robot colors in both legends match the corresponding bar colors.

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