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_compress.rs

@@ -702,18 +702,31 @@ pub fn build_compress_page(state: &AppState) -> adw::NavigationPage {
         });
     }
 
-    // Per-format slider handlers: update config, set preview to that format, refresh
+    // Per-slider debounce counters (separate to avoid cross-slider cancellation)
+    let jpeg_debounce: Rc<Cell<u32>> = Rc::new(Cell::new(0));
+    let png_debounce: Rc<Cell<u32>> = Rc::new(Cell::new(0));
+    let webp_debounce: Rc<Cell<u32>> = Rc::new(Cell::new(0));
+    let avif_debounce: Rc<Cell<u32>> = Rc::new(Cell::new(0));
+
+    // Per-format slider handlers: update config, set preview to that format, refresh (debounced)
     {
         let jc = state.job_config.clone();
         let row = jpeg_row.clone();
         let pc = preview_comp.clone();
         let up = update_preview.clone();
+        let did = jpeg_debounce.clone();
         jpeg_scale.connect_value_changed(move |scale| {
             let val = scale.value().round() as u8;
             jc.borrow_mut().jpeg_quality = val;
             row.set_subtitle(&format!("{}", val));
             pc.set(PreviewCompression::Jpeg(val));
-            up(false);
+            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(false); }
+            });
         });
     }
     {
@@ -721,12 +734,19 @@ pub fn build_compress_page(state: &AppState) -> adw::NavigationPage {
         let row = png_row.clone();
         let pc = preview_comp.clone();
         let up = update_preview.clone();
+        let did = png_debounce.clone();
         png_scale.connect_value_changed(move |scale| {
             let val = scale.value().round() as u8;
             jc.borrow_mut().png_level = val;
             row.set_subtitle(&format!("{}", val));
             pc.set(PreviewCompression::Png(val));
-            up(false);
+            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(false); }
+            });
         });
     }
     {
@@ -734,12 +754,19 @@ pub fn build_compress_page(state: &AppState) -> adw::NavigationPage {
         let row = webp_row.clone();
         let pc = preview_comp.clone();
         let up = update_preview.clone();
+        let did = webp_debounce.clone();
         webp_scale.connect_value_changed(move |scale| {
             let val = scale.value().round() as u8;
             jc.borrow_mut().webp_quality = val;
             row.set_subtitle(&format!("{}", val));
             pc.set(PreviewCompression::WebP(val));
-            up(false);
+            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(false); }
+            });
         });
     }
     {
@@ -756,12 +783,19 @@ pub fn build_compress_page(state: &AppState) -> adw::NavigationPage {
         let row = avif_row.clone();
         let pc = preview_comp.clone();
         let up = update_preview.clone();
+        let did = avif_debounce.clone();
         avif_scale.connect_value_changed(move |scale| {
             let val = scale.value().round() as u8;
             jc.borrow_mut().avif_quality = val;
             row.set_subtitle(&format!("{}", val));
             pc.set(PreviewCompression::Avif(val));
-            up(false);
+            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(false); }
+            });
         });
     }
     {