Add reMarkable backup system with web UI

- Docker container with rmapi cloud backup, PDF conversion pipeline
- Web UI: file browser, multi-select, delete/move, thumbnail preview
- Sync: backup from reMarkable cloud, rmdoc→PDF conversion via rmc+Inkscape
- Excluded-files mechanism to prevent deleted items from returning after sync

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

remarkable/web/Dockerfile

@@ -0,0 +1,7 @@
+FROM php:8.3-cli
+RUN apt-get update && apt-get install -y unzip && rm -rf /var/lib/apt/lists/*
+COPY backup/rmapi /usr/local/bin/rmapi
+RUN chmod +x /usr/local/bin/rmapi
+WORKDIR /app
+COPY web/ /app
+CMD ["php", "-S", "0.0.0.0:8080", "-t", "/app"]