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/components/DownloadSection.css

@@ -0,0 +1,40 @@
+.download-section {
+  background: rgba(186, 205, 176, 0.06);
+  border: 1px solid rgba(186, 205, 176, 0.12);
+  border-radius: 12px;
+  padding: 48px 40px;
+  text-align: center;
+}
+
+.download-title {
+  font-size: 1.5rem;
+  font-weight: 700;
+  color: #BACDB0;
+  margin: 0 0 12px;
+}
+
+.download-meta {
+  font-size: 0.85rem;
+  color: rgba(255, 255, 255, 0.45);
+  margin: 0 0 28px;
+}
+
+.btn-download {
+  display: inline-block;
+  padding: 16px 48px;
+  background: #BACDB0;
+  color: #2E2C2F;
+  border: none;
+  border-radius: 8px;
+  font-size: 1.1rem;
+  font-weight: 700;
+  text-decoration: none;
+  cursor: pointer;
+  transition: opacity 0.2s, transform 0.15s;
+  letter-spacing: 0.05em;
+}
+
+.btn-download:hover {
+  opacity: 0.9;
+  transform: translateY(-1px);
+}