Upgrade Next.js to 15, fix first-load theme bug, add SVG assets

- Upgrade next 14.1.0 → 15.5.9, next-themes 0.2.1 → 0.4.6
- Fix theme not initializing on first load after server restart
- Add SVG logos and favicon, update site-logo to use SVGs
- Create shared Header component with skip-to-main link
- Add portfolio links to bento grid cards
- Improve accessibility: aria-describedby, prefers-reduced-motion
- Fix useFormState → useActionState for React 19 compatibility
- Fix founder-signature icon (use square favicon instead of wide logo)

apps/web/package.json

@@ -14,8 +14,8 @@
     "@repo/ui": "workspace:*",
     "framer-motion": "^11.0.0",
     "lucide-react": "^0.350.0",
-    "next": "14.1.0",
-    "next-themes": "^0.2.1",
+    "next": "^15.1.3",
+    "next-themes": "^0.4.6",
     "react": "^18.2.0",
     "react-dom": "^18.2.0",
     "react-icons": "^5.5.0",
@@ -27,7 +27,7 @@
     "@types/react-dom": "^18.0.0",
     "autoprefixer": "^10.0.1",
     "eslint": "^8",
-    "eslint-config-next": "14.1.0",
+    "eslint-config-next": "^15.1.3",
     "postcss": "^8",
     "tailwindcss": "^3.3.0",
     "tailwindcss-animate": "^1.0.7",