Add per-step contextual help button in header bar

A "?" button in the header bar shows an AlertDialog with help
text specific to the current wizard step, explaining what each
step does and key shortcuts available.

pixstrip-gtk/src/app.rs

@@ -203,6 +203,14 @@ fn build_ui(app: &adw::Application) {
     let title = adw::WindowTitle::new("Pixstrip", "Batch Image Processor");
     header.set_title_widget(Some(&title));
 
+    // Help button for per-step contextual help
+    let help_button = gtk::Button::builder()
+        .icon_name("help-about-symbolic")
+        .tooltip_text("Help for this step")
+        .build();
+    help_button.add_css_class("flat");
+    header.pack_end(&help_button);
+
     // Hamburger menu
     let menu = build_menu();
     let menu_button = gtk::MenuButton::builder()
@@ -334,6 +342,16 @@ fn build_ui(app: &adw::Application) {
         state: app_state,
     };
 
+    // Wire help button to show contextual help for current step
+    {
+        let wizard = ui.state.wizard.clone();
+        let window_ref = window.clone();
+        help_button.connect_clicked(move |_| {
+            let step = wizard.borrow().current_step;
+            show_step_help(&window_ref, step);
+        });
+    }
+
     setup_window_actions(&window, &ui);
     update_nav_buttons(
         &ui.state.wizard.borrow(),
@@ -2203,6 +2221,86 @@ fn format_bytes(bytes: u64) -> String {
     }
 }
 
+fn show_step_help(window: &adw::ApplicationWindow, step: usize) {
+    let (title, body) = match step {
+        0 => ("Workflow", concat!(
+            "Choose a preset to start quickly, or configure each step manually.\n\n",
+            "Presets apply recommended settings for common tasks like web optimization, ",
+            "social media, or print preparation. You can customize any preset after applying it.\n\n",
+            "Use Import/Export to share presets with others."
+        )),
+        1 => ("Images", concat!(
+            "Add the images you want to process.\n\n",
+            "- Drag and drop files or folders onto this area\n",
+            "- Use Browse to pick files from a file dialog\n",
+            "- Press Ctrl+V to paste from clipboard\n\n",
+            "Use checkboxes to include or exclude individual images. ",
+            "Ctrl+A selects all, Ctrl+Shift+A deselects all."
+        )),
+        2 => ("Resize", concat!(
+            "Scale images to specific dimensions.\n\n",
+            "Choose a preset size or enter custom dimensions. Width-only or height-only ",
+            "resizing preserves the original aspect ratio.\n\n",
+            "Enable 'Allow upscale' if you need images smaller than the target to be enlarged."
+        )),
+        3 => ("Adjustments", concat!(
+            "Fine-tune image appearance.\n\n",
+            "Adjust brightness, contrast, and saturation with sliders. ",
+            "Apply rotation, flipping, grayscale, or sepia effects.\n\n",
+            "Crop to a specific aspect ratio or trim whitespace borders automatically."
+        )),
+        4 => ("Convert", concat!(
+            "Change image file format.\n\n",
+            "Convert between JPEG, PNG, WebP, AVIF, GIF, TIFF, and BMP. ",
+            "Each format has trade-offs between quality, file size, and compatibility.\n\n",
+            "WebP and AVIF offer the best compression for web use."
+        )),
+        5 => ("Compress", concat!(
+            "Reduce file size while preserving quality.\n\n",
+            "Choose a quality preset (Lossless, High, Balanced, Small, Tiny) or set custom ",
+            "quality values per format.\n\n",
+            "Expand Advanced Options for fine control over WebP encoding effort and AVIF speed."
+        )),
+        6 => ("Metadata", concat!(
+            "Control what metadata is kept or removed.\n\n",
+            "Strip All removes everything. Privacy mode keeps copyright and camera info but ",
+            "removes GPS and timestamps. Custom mode lets you pick exactly what to strip.\n\n",
+            "Removing metadata reduces file size and protects privacy."
+        )),
+        7 => ("Watermark", concat!(
+            "Add a text or image watermark.\n\n",
+            "Choose text or logo mode. Position the watermark using the visual grid. ",
+            "Expand Advanced Options for opacity, rotation, tiling, margin, and scale controls.\n\n",
+            "Logo watermarks support PNG images with transparency."
+        )),
+        8 => ("Rename", concat!(
+            "Rename output files using patterns.\n\n",
+            "Add a prefix, suffix, or use a full template with placeholders:\n",
+            "- {name} - original filename\n",
+            "- {n} - counter number\n",
+            "- {date} - current date\n",
+            "- {ext} - original extension\n\n",
+            "Expand Advanced Options for case conversion and find-and-replace."
+        )),
+        9 => ("Output", concat!(
+            "Review settings and choose where to save.\n\n",
+            "The summary shows all operations that will be applied. ",
+            "Choose an output folder or use the default 'processed' subfolder.\n\n",
+            "Set overwrite behavior for when output files already exist. ",
+            "Press Process or Ctrl+Enter to start."
+        )),
+        _ => ("Help", "No help available for this step."),
+    };
+
+    let dialog = adw::AlertDialog::builder()
+        .heading(format!("Help: {}", title))
+        .body(body)
+        .build();
+    dialog.add_response("ok", "Got it");
+    dialog.set_default_response(Some("ok"));
+    dialog.present(Some(window));
+}
+
 fn format_duration(ms: u64) -> String {
     if ms < 1000 {
         format!("{}ms", ms)