fix(ui): restore dark-mode contrast for prose table headers

[jfrench9]

Table headers rendered near-black on near-black wherever markdown goes through
the shared MarkdownProse — most visibly on Search, where results expand
into SEC narrative sections and XBRL text blocks that are full of tables.

Same fix as robosystems-app#351, which is where it was first spotted (in the
document viewer). This app was worse off: its typography.dark block stopped
at blockquote, with no table, marker or hr rules at all.

Cause

The typography plugin colors thead th from its --tw-prose-headings
variable:

.prose :where(thead th):not(...) { color: var(--tw-prose-headings); }

That selector outranks any thead element rule this config could set, so an
element-by-element dark variant can never recolor a header cell — while the
variable itself kept its light-mode default of near-black.

Fix

Override the --tw-prose-* variables in the dark variant, the way
prose-invert does, instead of chasing elements. Verified in the compiled CSS:
.dark\:prose-dark:is(.dark *) now emits --tw-prose-headings: #F9FAFB and,
at specificity (0,2,0), beats the base .prose (0,1,0) that set it near-black.

The variable values match this config's existing element rules, so nothing else
changes visually, and light mode is untouched. It also covers the
variable-driven elements this dark variant had no rules for at all: list
markers, hr, captions, kbd, and table borders.

npm run test:all green (368 tests).