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>

src/index.css

@@ -1,68 +1,33 @@
+*,
+*::before,
+*::after {
+  margin: 0;
+  padding: 0;
+  box-sizing: border-box;
+}
+
 :root {
-  font-family: system-ui, Avenir, Helvetica, Arial, sans-serif;
+  font-family: 'Pretendard', system-ui, -apple-system, sans-serif;
   line-height: 1.5;
   font-weight: 400;
-
-  color-scheme: light dark;
   color: rgba(255, 255, 255, 0.87);
-  background-color: #242424;
-
+  background-color: #2E2C2F;
   font-synthesis: none;
   text-rendering: optimizeLegibility;
   -webkit-font-smoothing: antialiased;
   -moz-osx-font-smoothing: grayscale;
 }
 
-a {
-  font-weight: 500;
-  color: #646cff;
-  text-decoration: inherit;
-}
-a:hover {
-  color: #535bf2;
-}
-
 body {
-  margin: 0;
-  display: flex;
-  place-items: center;
   min-width: 320px;
   min-height: 100vh;
 }
 
-h1 {
-  font-size: 3.2em;
-  line-height: 1.1;
+a {
+  color: #BACDB0;
+  text-decoration: none;
 }
 
-button {
-  border-radius: 8px;
-  border: 1px solid transparent;
-  padding: 0.6em 1.2em;
-  font-size: 1em;
-  font-weight: 500;
-  font-family: inherit;
-  background-color: #1a1a1a;
-  cursor: pointer;
-  transition: border-color 0.25s;
-}
-button:hover {
-  border-color: #646cff;
-}
-button:focus,
-button:focus-visible {
-  outline: 4px auto -webkit-focus-ring-color;
-}
-
-@media (prefers-color-scheme: light) {
-  :root {
-    color: #213547;
-    background-color: #ffffff;
-  }
-  a:hover {
-    color: #747bff;
-  }
-  button {
-    background-color: #f9f9f9;
-  }
+a:hover {
+  opacity: 0.85;
 }