fix(web): replace default markdown renderer and expand syntax support (#5689)

* fix(markdown): render default markdown with marked

- switch default frontend markdown rendering from react-markdown/remark-gfm to marked to avoid old WebKit parse failures from lookbehind regex literals
- sanitize marked HTML output with DOMPurify and preserve external link target and rel behavior
- remove default direct dependencies on react-markdown, remark-gfm, and rehype-raw while leaving classic unchanged

* fix(markdown): expand default markdown rendering support

- render default markdown with marked extensions for KaTeX formulas, page breaks, and common emoji shortcodes.
- sanitize KaTeX output with an explicit DOMPurify allowlist while preserving external link behavior.
- avoid overriding marked text rendering so lists and inline parsing keep their internal parser context.

* fix(markdown): render diagram code blocks in default UI

- add sanitized SVG rendering for flow and sequence diagram code blocks.
- size flow nodes from their labels and route edges from node anchors to prevent clipping.
- style diagram nodes, arrows, labels, and notes with theme-aware classes.

web/default/package.json

@@ -41,10 +41,13 @@
     "cmdk": "^1.1.1",
     "date-fns": "^4.3.0",
     "dayjs": "catalog:",
+    "dompurify": "3.4.5",
     "i18next": "^26.2.0",
     "i18next-browser-languagedetector": "^8.2.1",
     "input-otp": "^1.4.2",
+    "katex": "^0.17.0",
     "lucide-react": "^1.16.0",
+    "marked": "^18.0.5",
     "motion": "^12.40.0",
     "nanoid": "^5.1.11",
     "next-themes": "^0.4.6",
@@ -55,12 +58,9 @@
     "react-hook-form": "^7.76.1",
     "react-i18next": "^17.0.8",
     "react-icons": "catalog:",
-    "react-markdown": "catalog:",
     "react-resizable-panels": "^4.11.2",
     "react-top-loading-bar": "^3.0.2",
     "recharts": "3.8.1",
-    "rehype-raw": "^7.0.0",
-    "remark-gfm": "catalog:",
     "shiki": "^4.1.0",
     "sonner": "^2.0.7",
     "sse.js": "catalog:",