feat: add dark mode and audit viewer-frontend accessibility

Add next-themes light/dark/system theming with an accessible toggle
in every header, and close out an a11y pass across the viewer app:
skip-to-content link, main landmarks and heading hierarchy on every
route, focus-trapped dialog semantics for the photo viewer modals,
aria-label/live-region coverage for dynamic messages and icon
buttons, and dark: variants for previously light-only colors.

viewer-frontend/package-lock.json

@@ -27,6 +27,7 @@
         "lucide-react": "^0.553.0",
         "next": "^16.1.1",
         "next-auth": "^5.0.0-beta.30",
+        "next-themes": "^0.4.6",
         "nodemailer": "^7.0.13",
         "prisma": "^6.19.0",
         "react": "19.2.0",
@@ -6019,6 +6020,16 @@
         }
       }
     },
+    "node_modules/next-themes": {
+      "version": "0.4.6",
+      "resolved": "https://registry.npmjs.org/next-themes/-/next-themes-0.4.6.tgz",
+      "integrity": "sha512-pZvgD5L0IEvX5/9GWyHMf3m8BKiVQwsCMHfoFosXtXBMnaS0ZnIJ9ST4b4NqLVKDEm8QBxoNNGNaBv2JNF6XNA==",
+      "license": "MIT",
+      "peerDependencies": {
+        "react": "^16.8 || ^17 || ^18 || ^19 || ^19.0.0-rc",
+        "react-dom": "^16.8 || ^17 || ^18 || ^19 || ^19.0.0-rc"
+      }
+    },
     "node_modules/next/node_modules/postcss": {
       "version": "8.4.31",
       "funding": [