fix(hugo): preserve $...$ inline math through Goldmark passthrough

[inful]

Closes #62

Problem

Inline math written as $x+x$ was not being rendered by MathJax in
the Relearn theme, while display math $$x+x$$ rendered fine.

Root cause

The Goldmark passthrough extension is what preserves math delimiters
through markdown parsing and hands them to MathJax. In
internal/hugo/models/config.go, EnableMathPassthrough configured
only \\(...\\) as the inline delimiter set. $...$ was not in
the list, so Goldmark treated it as ordinary text and stripped/rewrote
it during rendering.

Fix

Add {"$", "$"} to the inline delimiters in
EnableMathPassthrough, alongside the existing \\(...\\) pair.
Both conventional inline-math forms now pass through to MathJax.
Display math delimiters ($$...$$, \\[...\\]) are unchanged
and were already working.

Trade-off: escaping literal $

With $...$ recognised as math, any paired $ characters in
prose will be treated as a math span. This is the same convention
KaTeX, MathJax and Pandoc use. Content with currency, shell variables,
or any other literal $ must escape with `\$. For example:

It costs \$5 and \$10.
Use \$HOME or \$TMPDIR for paths.

This is documented in docs/how-to/use-relearn-theme.md (new
"Math Support" section).

Verification

• go test ./... — all packages pass
• golangci-lint run ./... — 0 issues
• go vet ./internal/hugo/... — clean
• go build ./... — clean
• 18 golden test fixtures + 2 hugo_config testdata fixtures updated to
  reflect the new inline delimiter pair