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-core/tests/storage_tests.rs

@@ -208,7 +208,7 @@ fn add_and_list_history_entries() {
             ],
         };
 
-        history.add(entry.clone()).unwrap();
+        history.add(entry.clone(), 50, 30).unwrap();
         let entries = history.list().unwrap();
 
         assert_eq!(entries.len(), 1);
@@ -236,7 +236,7 @@ fn history_appends_entries() {
                     total_output_bytes: 500,
                     elapsed_ms: 100,
                     output_files: vec![],
-                })
+                }, 50, 30)
                 .unwrap();
         }
 
@@ -263,7 +263,7 @@ fn clear_history() {
                 total_output_bytes: 500,
                 elapsed_ms: 100,
                 output_files: vec![],
-            })
+            }, 50, 30)
             .unwrap();
 
         history.clear().unwrap();