fix(ci+chat): drop docker buildx flags + style chat code blocks

CI: gitea runner ships docker without the buildx plugin, so
`docker buildx build --platform linux/amd64 --load` exits 125
("unknown flag: --platform") in both the full build (build-stage.sh)
and the asset-only fast path (deploy.yaml). Runner host is already
linux/amd64, so explicit --platform is redundant. Switch both call
sites to plain `docker build`. This unblocks the pipeline — every
run since the workflow was added (~26 runs) has failed at this
exact step.

Chat: the live chat bubble renders marked.parse() output via
innerHTML, but .demo-msg-ai .demo-msg-bubble only had CSS rules
for p/ul/ol/li/strong. Fenced code blocks rendered as <pre><code>
with no styling — they appeared as wrapped plaintext, hard to
distinguish from prose. Add rules for code (inline and block),
pre, blockquote, em, h1-h4, and a, mirroring the share-card
styling (which always had them) so chat code blocks finally get
the monospace + tinted-background treatment users expect.

.gitea/workflows/deploy.yaml

@@ -146,7 +146,11 @@ jobs:
           COPY src/assets /srv/landing/assets
           COPY src/shares /srv/landing/shares
           EOF
-          docker buildx build --platform linux/amd64 --load \
+          # Plain `docker build` — the gitea runner ships docker without
+          # the buildx plugin, so `docker buildx build --platform ...`
+          # exits 125 ("unknown flag: --platform"). The runner host is
+          # already linux/amd64 so cross-platform isn't needed here.
+          docker build \
             --build-arg BASE="$LATEST" \
             -f /tmp/Dockerfile.assets \
             -t "marketing:${{ steps.tag.outputs.tag }}" \