feat: add backup file management settings

- Added settings UI to enable/disable .bak file creation when saving files
- Created settings storage in .obsidian/obsiviewer.json to persist backup preferences
- Added API endpoints to read/write backup settings (/api/settings)
- Implemented automatic cleanup of existing .bak files when disabling backups
- Modified file save operations to respect backup setting across notes, bookmarks and graph files
- Removed existing .bak files from version control

vault/Allo-3/test/Nouvelle note 3.md.bak

@@ -1,15 +0,0 @@
----
-titre: "Nouvelle note 3"
-auteur: "Bruno Charest"
-creation_date: "2025-10-24T15:44:07.120Z"
-modification_date: "2025-10-24T15:44:07.120Z"
-status: "en-cours"
-publish: false
-favoris: false
-template: false
-task: false
-archive: false
-draft: false
-private: false
----
-