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/preset_tests.rs

@@ -37,7 +37,7 @@ fn preset_serialization_roundtrip() {
 #[test]
 fn all_builtin_presets() {
     let presets = Preset::all_builtins();
-    assert_eq!(presets.len(), 8);
+    assert_eq!(presets.len(), 9);
     let names: Vec<&str> = presets.iter().map(|p| p.name.as_str()).collect();
     assert!(names.contains(&"Blog Photos"));
     assert!(names.contains(&"Social Media"));