fix: support nested Granola folders in folder-based path generation

[michaeljauk]

Summary

• Adds a folderIdMap (folder ID → folder object) when building folder mappings at sync time
• Updates generateFolderBasedPath to walk the parent_document_list_id chain, building the full nested path instead of always placing folders at the root of the sync directory
• Applies the configured filenameSeparator consistently to all path segments, including parent folder names

Problem

Granola supports nested spaces. Previously, only the direct folder name was used — the parent folder was silently dropped, so all sub-spaces were placed flat at the sync directory root.

Test plan

• Enable "Granola Folders" in plugin settings
• Verify that meetings in a nested space sync to <syncDir>/ParentSpace/ChildSpace/note.md
• Verify top-level spaces still sync to <syncDir>/SpaceName/note.md
• Verify meetings with no space still sync to <syncDir>/note.md
• Test with a non-default filenameSeparator — folder names should use it too