Update build system and path handling

- Use Node filesystem APIs instead of shell commands for temp cleanup
- Add absolute path helpers for cross-platform compatibility
- Generate both main.js and dist/main.js artifacts
- Improve error handling in document downloader
- Track additional document metadata in sync tracker
- Replace Unix-specific `find` with recursive directory traversal

The build system now generates two output files (main.js and dist/main.js) for
better compatibility with Obsidian's plugin loading. Path handling has been
centralized in new utilities to ensure cross-platform behavior, replacing shell
commands that had Windows compatibility issues.

package.json

@@ -2,7 +2,7 @@
   "name": "obsidian-remarkable",
   "version": "0.1.0",
   "description": "Obsidian plugin for reMarkable tablet sync and handwriting OCR",
-  "main": "main.ts",
+  "main": "main.js",
   "scripts": {
     "build": "node esbuild.config.mjs",
     "dev": "node esbuild.config.mjs --watch"