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

internal/cli/config.go

@@ -276,7 +276,8 @@ storage_url: ""
 # Hostname used in snapshot IDs. Default: system hostname.
 # hostname: myserver
 
-# Path to the local SQLite index database.
+# Path to the local SQLite index database. 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