Add tutorial overlay tour after welcome wizard

Semi-transparent dialog-based tour with 6 stops covering step
indicator, workflow selection, image adding, navigation, menu,
and a final ready message. Skippable at any time. State persisted
via tutorial_complete flag in AppConfig.

pixstrip-core/src/config.rs

@@ -4,6 +4,7 @@ use serde::{Deserialize, Serialize};
 #[serde(default)]
 pub struct AppConfig {
     pub first_run_complete: bool,
+    pub tutorial_complete: bool,
     pub output_subfolder: String,
     pub output_fixed_path: Option<String>,
     pub overwrite_behavior: OverwriteBehavior,
@@ -25,6 +26,7 @@ impl Default for AppConfig {
     fn default() -> Self {
         Self {
             first_run_complete: false,
+            tutorial_complete: false,
             output_subfolder: "processed".into(),
             output_fixed_path: None,
             overwrite_behavior: OverwriteBehavior::Ask,