feat: base volume path setting

Add global base_volume_path to settings. Relative volume source
paths are automatically prepended with the base path at deploy
time. Absolute paths are used as-is. Configurable in Settings >
General.

web/src/lib/types.ts

@@ -103,6 +103,7 @@ export interface Settings {
 	npm_password: string;
 	webhook_secret: string;
 	polling_interval: string;
+	base_volume_path: string;
 	updated_at: string;
 }