Fix 5 deferred performance/UX issues from audit

M8: Pre-compile regex once before rename preview loop instead of
recompiling per file. Adds apply_simple_compiled() to RenameConfig.

M9: Cache font data in watermark module using OnceLock (default font)
and Mutex<HashMap> (named fonts) to avoid repeated filesystem walks
during preview updates.

M12: Add 150ms debounce to watermark opacity, rotation, margin, and
scale sliders to avoid spawning preview threads on every pixel of
slider movement.

M13: Add 150ms debounce to compress per-format quality sliders (JPEG,
PNG, WebP, AVIF) for the same reason.

M14: Move thumbnail loading to background threads instead of blocking
the GTK main loop. Each thumbnail is decoded via image crate in a
spawned thread and delivered to the main thread via channel polling.

pixstrip-gtk/src/steps/step_watermark.rs

@@ -686,19 +686,32 @@ pub fn build_watermark_page(state: &AppState) -> adw::NavigationPage {
         });
     }
 
+    // Per-slider debounce counters (separate to avoid cross-slider cancellation)
+    let opacity_debounce: Rc<Cell<u32>> = Rc::new(Cell::new(0));
+    let rotation_debounce: Rc<Cell<u32>> = Rc::new(Cell::new(0));
+    let margin_debounce: Rc<Cell<u32>> = Rc::new(Cell::new(0));
+    let scale_debounce: Rc<Cell<u32>> = Rc::new(Cell::new(0));
+
     // Opacity slider
     {
         let jc = state.job_config.clone();
         let row = opacity_row.clone();
         let up = update_preview.clone();
         let rst = opacity_reset.clone();
+        let did = opacity_debounce.clone();
         opacity_scale.connect_value_changed(move |scale| {
             let val = scale.value().round() as i32;
             let opacity = val as f32 / 100.0;
             jc.borrow_mut().watermark_opacity = opacity;
             row.set_subtitle(&format!("{}%", val));
             rst.set_sensitive(val != 50);
-            up();
+            let up = up.clone();
+            let did = did.clone();
+            let id = did.get().wrapping_add(1);
+            did.set(id);
+            glib::timeout_add_local_once(std::time::Duration::from_millis(150), move || {
+                if did.get() == id { up(); }
+            });
         });
     }
     {
@@ -714,12 +727,19 @@ pub fn build_watermark_page(state: &AppState) -> adw::NavigationPage {
         let row = rotation_row.clone();
         let up = update_preview.clone();
         let rst = rotation_reset.clone();
+        let did = rotation_debounce.clone();
         rotation_scale.connect_value_changed(move |scale| {
             let val = scale.value().round() as i32;
             jc.borrow_mut().watermark_rotation = val;
             row.set_subtitle(&format!("{} degrees", val));
             rst.set_sensitive(val != 0);
-            up();
+            let up = up.clone();
+            let did = did.clone();
+            let id = did.get().wrapping_add(1);
+            did.set(id);
+            glib::timeout_add_local_once(std::time::Duration::from_millis(150), move || {
+                if did.get() == id { up(); }
+            });
         });
     }
     {
@@ -745,12 +765,19 @@ pub fn build_watermark_page(state: &AppState) -> adw::NavigationPage {
         let row = margin_row.clone();
         let up = update_preview.clone();
         let rst = margin_reset.clone();
+        let did = margin_debounce.clone();
         margin_scale.connect_value_changed(move |scale| {
             let val = scale.value().round() as i32;
             jc.borrow_mut().watermark_margin = val as u32;
             row.set_subtitle(&format!("{} px", val));
             rst.set_sensitive(val != 10);
-            up();
+            let up = up.clone();
+            let did = did.clone();
+            let id = did.get().wrapping_add(1);
+            did.set(id);
+            glib::timeout_add_local_once(std::time::Duration::from_millis(150), move || {
+                if did.get() == id { up(); }
+            });
         });
     }
     {
@@ -766,12 +793,19 @@ pub fn build_watermark_page(state: &AppState) -> adw::NavigationPage {
         let row = scale_row.clone();
         let up = update_preview.clone();
         let rst = scale_reset.clone();
+        let did = scale_debounce.clone();
         scale_scale.connect_value_changed(move |scale| {
             let val = scale.value().round() as i32;
             jc.borrow_mut().watermark_scale = val as f32;
             row.set_subtitle(&format!("{}%", val));
             rst.set_sensitive((val - 20).abs() > 0);
-            up();
+            let up = up.clone();
+            let did = did.clone();
+            let id = did.get().wrapping_add(1);
+            did.set(id);
+            glib::timeout_add_local_once(std::time::Duration::from_millis(150), move || {
+                if did.get() == id { up(); }
+            });
         });
     }
     {