fix(ui): use brand color tokens instead of hardcoded hex in new UI

[ksitton58]

Description

Several new-UI templates hardcoded brand colors that should reference the design
tokens in components/tailwind.css:

• calendar.html — active engagement/test events used the legacy Bootstrap primary
  blue (#337ab7) instead of the brand blue.
• benchmark.html — table link color, same legacy #337ab7.
• base.html — sidebar background (#003864) and sub-nav link color (#82B0D9)
  hardcoded the Fuji Blue brand hues.

These now use the matching var(--color-dd-primary-*) tokens (500 / 900 / 200). The
token values are identical to the hardcoded hex, so there is no visual change.

Intentionally left as-is:

• Sidebar's lighter link color (#C6DDF2): the matching shade (dd-primary-100) is
  not emitted in the compiled CSS, since Tailwind v4 only outputs theme variables
  that a generated utility references.
• PDF report templates also contain #337ab7, but CSS custom properties don't resolve
  in the PDF renderer.
• Generic white/black, neutral grays, and the custom #002a4d shade have no token.

FullCalendar 3.10.5 applies the event color verbatim as an inline style, so the
variable resolves at render time.

Test results

Verified in the running new UI: sidebar bg = rgb(0,56,100), sub-nav link =
rgb(130,176,217), calendar event = rgb(23,121,197) — each identical to the hex it
replaced. No Python changed, so no unit tests added.

Documentation

N/A.

Checklist

• Submitted against dev.
• Meaningful PR name.
• Ruff compliant (no Python changed).
• Python 3.13 compliant (no Python changed).