Fix 26 bugs, edge cases, and consistency issues from fifth audit pass

Critical: undo toast now trashes only batch output files (not entire dir),
JPEG scanline write errors propagated, selective metadata write result returned.

High: zero-dimension guards in ResizeConfig/fit_within, negative aspect ratio
rejection, FM integration toggle infinite recursion guard, saturating counter
arithmetic in executor.

Medium: PNG compression level passed to oxipng, pct mode updates job_config,
external file loading updates step indicator, CLI undo removes history entries,
watch config write failures reported, fast-copy path reads image dimensions for
rename templates, discovery excludes unprocessable formats (heic/svg/ico/jxl),
CLI warns on invalid algorithm/overwrite values, resolve_collision trailing dot
fix, generation guards on all preview threads to cancel stale results, default
DPI aligned to 0, watermark text width uses char count not byte length.

Low: binary path escaped in Nautilus extension, file dialog filter aligned with
discovery, reset_wizard clears preset_mode and output_dir.

pixstrip-gtk/src/processing.rs

@@ -88,15 +88,10 @@ pub fn build_processing_page() -> adw::NavigationPage {
     content.append(&log_group);
     content.append(&button_box);
 
-    let clamp = adw::Clamp::builder()
-        .maximum_size(600)
-        .child(&content)
-        .build();
-
     adw::NavigationPage::builder()
         .title("Processing")
         .tag("processing")
-        .child(&clamp)
+        .child(&content)
         .build()
 }
 
@@ -232,14 +227,9 @@ pub fn build_results_page() -> adw::NavigationPage {
 
     scrolled.set_child(Some(&content));
 
-    let clamp = adw::Clamp::builder()
-        .maximum_size(600)
-        .child(&scrolled)
-        .build();
-
     adw::NavigationPage::builder()
         .title("Results")
         .tag("results")
-        .child(&clamp)
+        .child(&scrolled)
         .build()
 }