Add CI workflow, full env docs, fix Up button navigation

- .gitea/workflows/build.yaml: Gitea Actions — push latest on main
- docker-compose.example.yml: image-based pull, all NAT20_ vars shown
- settings.py: add NAT20_PLAYER_RECAP_STYLE / CUSTOM_PROMPT overrides
- README.md: full env var reference table with accepted values + rationale
- files.py: fix parent_path to avoid up-button bug; add path traversal guard
- Files.tsx: remove copy/paste/autoPlay, scope to campaign

.gitea/workflows/build.yaml

@@ -0,0 +1,29 @@
+name: Build and Push
+on:
+  push:
+    branches: [main]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: docker/setup-buildx-action@v3
+
+      - name: Login to Gitea Container Registry
+        run: echo "${{ secrets.GITEA_TOKEN }}" | docker login gitea.kansaigaijin.com -u ${{ gitea.repository_owner }} --password-stdin
+
+      - name: Build and push backend
+        uses: docker/build-push-action@v5
+        with:
+          context: ./backend
+          target: runtime
+          push: true
+          tags: gitea.kansaigaijin.com/${{ gitea.repository }}/backend:latest
+
+      - name: Build and push frontend
+        uses: docker/build-push-action@v5
+        with:
+          context: ./frontend
+          push: true
+          tags: gitea.kansaigaijin.com/${{ gitea.repository }}/frontend:latest