Per-series color & dashed line style for ui_line_chart

[tylercschneider]

Closes #137

What

Adds per-series styling to ui_line_chart so overlaid lines (e.g. current vs previous period) are visually distinguishable.

A series entry now accepts two optional keys:

• color: — any CSS color, or an accent token like var(--color-accent-500).
• dashed: — true renders a dashed line (borderDash: [6, 6]).

<%= ui_line_chart(labels: %w[Mon Tue Wed], series: [
  { name: "This period", data: [3, 5, 4] },
  { name: "Prev period",  data: [2, 3, 3], color: "var(--color-accent-500)", dashed: true }
]) %>

Notes

• Backward compatible — a bare { name:, data: } series produces the exact same dataset as before; the styling keys are only added when present.
• Accent/surface palette — Chart.js draws on canvas and can't resolve var(--color-...) strings, so the line-chart controller resolves CSS custom-property borderColors to concrete values at connect time. This lets the accent/surface color system drive line colors.
• Documented in the reference guide + CHANGELOG; committed the_local locals rebuilt (rake the_local:build).

Tests

• Series color: → dataset borderColor.
• Series dashed: true → dataset borderDash.
• Existing suite green (426 runs).

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