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.

docs/docs.json

@@ -4,8 +4,8 @@
   "name": "Sencho",
   "colors": {
     "primary": "#00BEC7",
-    "light": "#0891B2",
-    "dark": "#22D3EE"
+    "light": "#007982",
+    "dark": "#00BEC7"
   },
   "logo": {
     "light": "/images/logo/logo-light.png",
@@ -22,7 +22,7 @@
   "background": {
     "decoration": "gradient",
     "color": {
-      "dark": "#0F172A"
+      "dark": "#090909"
     }
   },
   "navbar": {