Wire welcome dialog with navigation and first-run detection

- Welcome dialog buttons navigate between pages
- Done button closes dialog and marks first_run_complete
- Show welcome dialog on first launch only
- Add first_run_complete field to AppConfig with serde(default)

pixstrip-core/src/config.rs

@@ -1,7 +1,9 @@
 use serde::{Deserialize, Serialize};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(default)]
 pub struct AppConfig {
+    pub first_run_complete: bool,
     pub output_subfolder: String,
     pub output_fixed_path: Option<String>,
     pub overwrite_behavior: OverwriteBehavior,
@@ -20,6 +22,7 @@ pub struct AppConfig {
 impl Default for AppConfig {
     fn default() -> Self {
         Self {
+            first_run_complete: false,
             output_subfolder: "processed".into(),
             output_fixed_path: None,
             overwrite_behavior: OverwriteBehavior::Ask,