Add frontend pages with login, download, and announcements

- React Router v7: public home page, /login page
- Auth context with JWT localStorage management
- Login: ID/PW form + SSAFY login button (UI only)
- Home: hero banner, download section (login required), announcement board
- API layer with mock data (ready for Go Fiber backend)
- Color scheme: #2E2C2F dark + #BACDB0 accent
- Add .env.example for environment variable reference

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package-lock.json

@@ -9,7 +9,8 @@
       "version": "0.0.0",
       "dependencies": {
         "react": "^19.2.0",
-        "react-dom": "^19.2.0"
+        "react-dom": "^19.2.0",
+        "react-router-dom": "^7.13.1"
       },
       "devDependencies": {
         "@eslint/js": "^9.39.1",
@@ -1571,6 +1572,19 @@
       "dev": true,
       "license": "MIT"
     },
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+      "resolved": "https://registry.npmjs.org/cookie/-/cookie-1.1.1.tgz",
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+      "license": "MIT",
+      "engines": {
+        "node": ">=18"
+      },
+      "funding": {
+        "type": "opencollective",
+        "url": "https://opencollective.com/express"
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     "node_modules/cross-spawn": {
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       "resolved": "https://registry.npmjs.org/cross-spawn/-/cross-spawn-7.0.6.tgz",
@@ -2410,6 +2424,44 @@
         "node": ">=0.10.0"
       }
     },
+    "node_modules/react-router": {
+      "version": "7.13.1",
+      "resolved": "https://registry.npmjs.org/react-router/-/react-router-7.13.1.tgz",
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+      "license": "MIT",
+      "dependencies": {
+        "cookie": "^1.0.1",
+        "set-cookie-parser": "^2.6.0"
+      },
+      "engines": {
+        "node": ">=20.0.0"
+      },
+      "peerDependencies": {
+        "react": ">=18",
+        "react-dom": ">=18"
+      },
+      "peerDependenciesMeta": {
+        "react-dom": {
+          "optional": true
+        }
+      }
+    },
+    "node_modules/react-router-dom": {
+      "version": "7.13.1",
+      "resolved": "https://registry.npmjs.org/react-router-dom/-/react-router-dom-7.13.1.tgz",
+      "integrity": "sha512-UJnV3Rxc5TgUPJt2KJpo1Jpy0OKQr0AjgbZzBFjaPJcFOb2Y8jA5H3LT8HUJAiRLlWrEXWHbF1Z4SCZaQjWDHw==",
+      "license": "MIT",
+      "dependencies": {
+        "react-router": "7.13.1"
+      },
+      "engines": {
+        "node": ">=20.0.0"
+      },
+      "peerDependencies": {
+        "react": ">=18",
+        "react-dom": ">=18"
+      }
+    },
     "node_modules/rollup": {
       "version": "4.59.0",
       "resolved": "https://registry.npmjs.org/rollup/-/rollup-4.59.0.tgz",
@@ -2471,6 +2523,12 @@
         "semver": "bin/semver.js"
       }
     },
+    "node_modules/set-cookie-parser": {
+      "version": "2.7.2",
+      "resolved": "https://registry.npmjs.org/set-cookie-parser/-/set-cookie-parser-2.7.2.tgz",
+      "integrity": "sha512-oeM1lpU/UvhTxw+g3cIfxXHyJRc/uidd3yK1P242gzHds0udQBYzs3y8j4gCCW+ZJ7ad0yctld8RYO+bdurlvw==",
+      "license": "MIT"
+    },
     "node_modules/shebang-command": {
       "version": "2.0.0",
       "resolved": "https://registry.npmjs.org/shebang-command/-/shebang-command-2.0.0.tgz",