deps: fix the npm audit advisories that turned CI red (#345)

`npm audit --audit-level=high --omit=dev` (Build & Quality Checks) started
failing on every branch after new advisories were published:

- source-map-js 1.2.1 → 1.2.2 (high, GHSA-68fv-2mgg-jv7q; transitive via
  sanitize-html → postcss, in range — lockfile only)
- i18next-http-backend 4.0.0 → 4.0.2 (low, GHSA-xvq9-wjp8-hwqf)
- katex 0.16.47 → 0.18.11 (low, GHSA-238p-pmpm-9mq7; fixed in 0.18.2).
  0.17/0.18's breaking changes are an internal __defineFunction API and
  prefixed internal CSS classes; Lotus uses neither (KaTeX.tsx calls
  renderToString and imports the package's own stylesheet).

Only these four lockfile entries change (plus katex's own CLI dependency
commander 8 → 15). npm audit --omit=dev: 0 vulnerabilities.

Verified headless against Vite with a fresh dep cache (katex 0.18.11
served): inline and display math render (\frac, \sqrt, \sum with limits),
KaTeX fonts load, no page errors; translations still load (en strings,
no raw keys). tsc clean, 1319 unit tests pass, eslint 0 errors (36
warnings, unchanged), prettier clean, production build OK.

Fixes #345

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

package.json

@@ -72,12 +72,12 @@
     "html-react-parser": "6.1.2",
     "i18next": "26.2.0",
     "i18next-browser-languagedetector": "8.2.1",
-    "i18next-http-backend": "4.0.0",
+    "i18next-http-backend": "4.0.2",
     "immer": "11.1.8",
     "is-hotkey": "0.2.0",
     "jotai": "2.20.0",
     "jsqr": "1.4.0",
-    "katex": "0.16.47",
+    "katex": "0.18.11",
     "linkify-react": "4.3.3",
     "linkifyjs": "4.3.3",
     "matrix-js-sdk": "41.7.0",