Fix 16 medium-severity bugs from audit

CLI: add UTC suffix to timestamps, validate image extensions on
single-file input, canonicalize watch paths for reliable matching,
derive counter_enabled from template presence, warn when undo count
exceeds available batches.

Core: apply space/special-char transforms in template rename path,
warn on metadata preservation for unsupported formats, derive AVIF
speed from compress preset quality level.

GTK: use buffer size for apples-to-apples compress preview comparison,
shorten approximate format labels, cache file sizes to avoid repeated
syscalls on checkbox toggle, add batch-update guard to prevent O(n^2)
in select/deselect all, use widget names for reliable progress/log
lookup, add unique suffix for duplicate download filenames.

pixstrip-core/src/preset.rs

@@ -63,7 +63,10 @@ impl Preset {
             overwrite_behavior: crate::operations::OverwriteAction::default(),
             preserve_directory_structure: false,
             progressive_jpeg: false,
-            avif_speed: 6,
+            avif_speed: self.compress.as_ref().map(|c| match c {
+                crate::operations::CompressConfig::Preset(p) => p.avif_speed(),
+                _ => 6,
+            }).unwrap_or(6),
             output_dpi: 72,
         }
     }