docs: align documentation theme with Sencho app branding (#199)

* docs: align documentation theme with Sencho app branding

- Change dark background from #0F172A (blue-tinted slate) to #090909 (neutral dark)
- Update accent colors to match app brand oklch(0.72 0.14 200)
- Add custom.css to increase navbar and footer logo size from 28px to 36px

* fix(deps): resolve npm audit vulnerabilities in backend

Fix path-to-regexp (high) and brace-expansion (moderate) via npm audit fix.

backend/package-lock.json

@@ -1551,16 +1551,16 @@
       }
     },
     "node_modules/brace-expansion": {
-      "version": "5.0.2",
-      "resolved": "https://registry.npmjs.org/brace-expansion/-/brace-expansion-5.0.2.tgz",
-      "integrity": "sha512-Pdk8c9poy+YhOgVWw1JNN22/HcivgKWwpxKq04M/jTmHyCZn12WPJebZxdjSa5TmBqISrUSgNYU3eRORljfCCw==",
+      "version": "5.0.5",
+      "resolved": "https://registry.npmjs.org/brace-expansion/-/brace-expansion-5.0.5.tgz",
+      "integrity": "sha512-VZznLgtwhn+Mact9tfiwx64fA9erHH/MCXEUfB/0bX/6Fz6ny5EGTXYltMocqg4xFAQZtnO3DHWWXi8RiuN7cQ==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
         "balanced-match": "^4.0.2"
       },
       "engines": {
-        "node": "20 || >=22"
+        "node": "18 || 20 || >=22"
       }
     },
     "node_modules/braces": {
@@ -3981,9 +3981,9 @@
       }
     },
     "node_modules/path-to-regexp": {
-      "version": "8.3.0",
-      "resolved": "https://registry.npmjs.org/path-to-regexp/-/path-to-regexp-8.3.0.tgz",
-      "integrity": "sha512-7jdwVIRtsP8MYpdXSwOS0YdD0Du+qOoF/AEPIt88PcCFrZCzx41oxku1jD88hZBwbNUIEfpqvuhjFaMAqMTWnA==",
+      "version": "8.4.0",
+      "resolved": "https://registry.npmjs.org/path-to-regexp/-/path-to-regexp-8.4.0.tgz",
+      "integrity": "sha512-PuseHIvAnz3bjrM2rGJtSgo1zjgxapTLZ7x2pjhzWwlp4SJQgK3f3iZIQwkpEnBaKz6seKBADpM4B4ySkuYypg==",
       "license": "MIT",
       "funding": {
         "type": "opencollective",