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/Cargo.toml

@@ -9,3 +9,4 @@ pixstrip-core = { workspace = true }
 gtk = { package = "gtk4", version = "0.11.0", features = ["gnome_49"] }
 adw = { package = "libadwaita", version = "0.9.1", features = ["v1_8"] }
 image = "0.25"
+regex = "1"