fix(markup): use installed math fonts for MathML in Chromium (#39491)

On Linux and ChromeOS, Chromium resolves the `math` font family to Latin
Modern Math, which neither installs, so MathML renders with a text font
and brackets and large operators don't stretch, see
https://issues.chromium.org/issues/40069293. The new `--fonts-math`
variable keeps `math` first, so browsers that always resolve it keep
their font. Only Chromium falls through to the math fonts Linux and
ChromeOS install by default:

- `STIX Two Math`: Fedora, and `fonts-stix` on Ubuntu 26.04
- `DejaVu Math TeX Gyre`: Debian 13 and openSUSE
- `Noto Sans Math`: Fedora and ChromeOS, last because Debian and Ubuntu
ship an older version without a `MATH` table

Math fonts also have smaller x-heights than UI fonts, so MathML rendered
smaller than KaTeX and the surrounding text in every browser, see
https://github.com/w3c/mathml-core/issues/41. `font-size-adjust:
ex-height 0.52` scales whichever math font is used to KaTeX's x-height.
KaTeX output is unchanged.

Fixes: https://github.com/go-gitea/gitea/issues/39489
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: bircni <75789103+bircni@users.noreply.github.com>
Co-authored-by: silverwind <me@silverwind.io>

web_src/css/base.css

@@ -2,6 +2,8 @@
   /* fonts */
   --fonts-proportional: -apple-system, "Segoe UI", system-ui, Roboto, "Helvetica Neue", Arial;
   --fonts-monospace: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace, var(--fonts-emoji);
+  /* Chromium's "math" is a font Linux and ChromeOS don't ship, so fall back to the math fonts they do, https://issues.chromium.org/issues/40069293 */
+  --fonts-math: math, "STIX Two Math", "DejaVu Math TeX Gyre", "Noto Sans Math";
   /* GitHub explicitly sets font names like: "Apple Color Emoji", "Segoe UI Emoji", "Noto Color Emoji", "Twemoji Mozilla";
   Actually "Twemoji Mozilla" emoji font is widely used by browsers like Firefox, Pale Moon, and it is more likely up-to-dated than the system emoji font.
   So not setting emoji font seems to be the best choice, here we just use a non-existing dummy font name and let browsers choose. */
@@ -107,6 +109,11 @@ samp,
   font-size: 0.95em; /* compensate for monospace fonts being usually slightly larger */
 }
 
+math {
+  font-family: var(--fonts-math);
+  font-size-adjust: ex-height 0.52; /* match KaTeX's x-height (0.431 × 1.21em) regardless of math font, https://github.com/w3c/mathml-core/issues/41 */
+}
+
 /* there are many <code> blocks in non-markup(.markup code) / non-code-diff(code.code-inner) containers, for example: translation strings, etc,
 so we need to make <code> have default global styles, ".markup code" has its own styles and these styles sometimes conflict.
 TODO: in the future, we should use `div` instead of `code` for `.code-inner` because it is a container for highlighted code line, then drop this ":not" patch */