fix Tailwind 4 production build OOM by scoping source detection

Tailwind 4's automatic source detection scans from the current working
directory. The Docker frontend stage has no WORKDIR, so cwd is `/` and
Tailwind tries to ingest the entire container filesystem. That work
happens in oxide/lightningcss native (off-heap) code, so the build ran
~11 minutes and was then OOM-killed -- and raising --max-old-space-size
never helped, because that only bounds the V8 heap, not native memory.

Disable auto-detection with `source(none)` and declare the sources
explicitly instead (mirroring the previous tailwind.config.js `content`
globs, including .py for classes used in Wagtail blocks/widgets). The
theme is loaded explicitly via @config.

Frontend docker build (npm run build:prod): 651s + OOM -> ~3s.

Author: AJ Griffiths

tbx/static_src/sass/main.scss

@@ -93,7 +93,15 @@
 
 // Layout
 
-@import 'tailwindcss';
+// `source(none)` disables Tailwind 4's automatic source detection, which
+// otherwise scans from the current working directory — that is `/` inside the
+// Docker build stage (no WORKDIR), so it tries to ingest the whole container
+// filesystem and OOM-hangs the build. We instead load the JS config and list
+// the source globs explicitly (paths are relative to this file).
+@import 'tailwindcss' source(none);
+@config '../../../tailwind.config.js';
+@source '../../**/*.{py,html}';
+@source '../**/*.{js,ts,tsx,vue}';
 
 @utility container {
     margin-inline: auto;