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/src/discovery.rs

@@ -3,8 +3,7 @@ use std::path::{Path, PathBuf};
 use walkdir::WalkDir;
 
 const IMAGE_EXTENSIONS: &[&str] = &[
-    "jpg", "jpeg", "png", "webp", "avif", "gif", "tiff", "tif", "bmp", "heic", "heif", "jxl",
-    "svg", "ico",
+    "jpg", "jpeg", "png", "webp", "avif", "gif", "tiff", "tif", "bmp",
 ];
 
 fn is_image_extension(ext: &str) -> bool {