feat(chart): primary shades for parts of one total

[zyx1121]

From migrating llm.winlab.tw (NYCU-WinLab/llm.winlab.tw#80): its daily usage chart stacks up to six models plus 其他 and 未路由. DESIGN.md allowed only two colored series, and a stack cannot be split into small multiples. Loki decided (2026-10-14) to make shades of primary the rule.

• ChartColor adds primary-2 to primary-6: color-mix(in oklch, var(--primary) N%, var(--background)), so the steps hold in both themes; primaryShades lists them darkest first
• DESIGN.md Color: a new row and the exception to the two-series rule (stack, largest first, at most six, then 其他 in muted-foreground)
• The ChartConfig comment says keys must be CSS names; the demo keys series by position (a key like qwen3.8 made its bar black, found while checking this)
• Chart demo: a second chart, tokens per day stacked by model

Checked on sandbox: lint, typecheck, format, tokens, build; screenshots of /chart in light and dark show four distinct steps.

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