Refuse a relative index_path (closes #430)

A relative index_path, or VAULTIK_INDEX_PATH, was accepted and resolved against whatever directory vaultik ran in. Loading the config now refuses a relative or empty value, quoting it, with the same wording as the file:// check; ~ is still expanded first.

Model: opus-5-5

configs/config.example.yml

@@ -295,6 +295,7 @@ storage_url: "rclone://myremote/path/to/backups"
 
 # Path to local SQLite index database
 # This database tracks file state for incremental backups
+# It must be an absolute path; ~ is expanded
 # Default: the platform data directory, e.g.
 #   macOS: ~/Library/Application Support/vaultik/index.sqlite
 #   Linux: ~/.local/share/vaultik/index.sqlite