Use primary-container token instead of hardcoded hex on About card

[Kanyandula]

Summary

• Replace bg-[#0f3d3e] on the About-page founder card with the
  bg-primary-container semantic token.
• Removes a hardcoded-hex convention violation (CLAUDE.md: dark mode via
  CSS custom properties, no hardcoded colors).

Visual impact

• Light mode: pixel-identical — --color-primary-container light is
  exactly #0f3d3e.
• Dark mode: card now resolves to #1a4f50 and adapts with the
  palette instead of staying a fixed light-mode teal slab. White body
  text (text-white / text-white/80) stays ≈8.7:1 on it (AAA).
• Template-only change; compiled CSS already contained
  .bg-primary-container, so no asset rebuild.

Coordination note

Related open PR fix/about-founder-title-contrast edits the adjacent
line (the card's <h2>). Diff contexts overlap, so depending on merge
order a trivial conflict may surface — resolution is simply keeping both
one-line changes (line 32 token swap + line 33 text-white).

Test plan

• Load /about in light mode — founder card unchanged.
• Toggle dark mode — card background adapts (teal #1a4f50), title
  and body remain readable.

[Kanyandula]

Merge ordering: merge #58 before this PR.

#58 (fix/about-founder-title-contrast) is already live in production
and only needs master to catch up; it edits line 33 of
personal/templates/personal/about.html while this PR edits line 32.
Overlapping diff context means whichever merges second may hit a trivial
conflict — resolve by keeping both one-line changes (text-white on
the <h2> from #58 + bg-primary-container here).