fix(webui): default fenceless code blocks to plain text rendering

Language-less ASCII-art fences (diagrams, terminal output, plain text) were
being heuristically tokenised by shiki/rehype-pretty-code, producing stripey
per-line backgrounds and broken alignment because the highlighter guessed at
keywords that were not there.

Set rehype-pretty-code's `defaultLang: 'text'` so any fenced block without an
explicit language renders as a uniform monospace block. Source markdown stays
unchanged — no doc-side annotations needed.

Verified locally against docs/agent/authentication.md: the client_credentials
diagram now renders as a single contiguous block.

See also #219 (view-source link on docs pages) and #220 (rewrite relative .md
links in webui rendering).

Author: vredchenko

webui/vite.config.ts

@@ -81,6 +81,7 @@ export default defineConfig({
           {
             theme: 'github-dark',
             keepBackground: true,
+            defaultLang: 'text',
           },
         ],
       ],