fix(frontend): resolve Snyk security vulnerabilities on release-2.8

Address 19 of 20 targeted security vulnerabilities identified by Snyk
across the frontend dependency tree.

Changes:
- Replace node-polyfill-webpack-plugin with @rsbuild/plugin-node-polyfill
  (eliminates pbkdf2 and sha.js critical CVEs at the source)
- Remove unused remark-prism dependency
  (eliminates form-data critical, ws high, and prismjs low CVEs)
- Add dependency overrides for transitive vulnerability fixes:
  - pbkdf2@3.1.5 (CVE-2025-6547, CVE-2025-6545)
  - sha.js@2.4.12 (CVE-2025-9288)
  - @remix-run/node@2.17.4 (CVE-2025-61686)
  - @remix-run/router@1.23.2 (CVE-2026-22029)
  - @babel/runtime@7.28.6 (CVE-2025-27789)
  - prismjs@1.30.0 (CVE-2024-53382)
  - path-to-regexp@1.9.0 (CVE-2024-45296)

Not fixed: mdast-util-to-hast@13.2.0 (1 medium) — override to 13.2.1
breaks @redpanda-data/ui which depends on the removed 'all' export.
Requires a @redpanda-data/ui library upgrade.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: malinskibeniamin