Add reMarkable browser modal with import and conversion options

The browser modal allows users to navigate their reMarkable file structure, view documents and folders, and perform
actions on individual files. Users can either import documents directly or convert handwriting to Markdown during
import.

The implementation includes:
- New ribbon icon and command to open the browser
- Modal UI with navigation controls (up/refresh)
- Document listing with folder traversal
- Per-file actions: Import and Handwriting to Markdown
- Proper error handling and user feedback
- Comprehensive tests for the new functionality

The downloader was also enhanced to support the new conversion options through a more flexible interface.

IMPLEMENTATION.md

@@ -97,6 +97,8 @@ obidian-remarkable/
 
 ### Daily Use
 - **Sync all**: Click ribbon pencil icon or run command "Sync from reMarkable"
+- **Browse files**: Click ribbon folder icon or run command "Browse reMarkable"
+- **Per-file actions**: In the browser, click "Import" or "Handwriting to Markdown" for each document
 - **Convert single file**: Open a `.rm` file in Obsidian, run "Convert handwriting to Markdown"
 
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