Fix 30 critical and high severity bugs from audit passes 6-8

Critical fixes:
- Prevent path traversal via rename templates (sanitize_filename)
- Prevent input == output data loss (paths_are_same check)
- Undo now uses actual executor output paths instead of scanning directory
- Filter empty paths from output_files (prevents trashing CWD on undo)
- Sanitize URL download filenames to prevent path traversal writes

High severity fixes:
- Fix EXIF orientation 5/7 transforms per spec
- Atomic file creation in find_unique_path (TOCTOU race)
- Clean up 0-byte placeholder files on encoding failure
- Cap canvas padding to 10000px, total dimensions to 65535
- Clamp crop dimensions to minimum 1px
- Clamp DPI to 65535 before u16 cast in JPEG encoder
- Force pixel path for non-JPEG/TIFF metadata stripping
- Fast path now applies regex find/replace on rename stem
- Add output_dpi to needs_pixel_processing check
- Cap watermark image scale dimensions to 16384
- Cap template counter padding to 10
- Cap URL download size to 100MB
- Fix progress bar NaN when total is zero
- Fix calculate_eta underflow when current > total
- Fix loaded.len()-1 underflow in preview callbacks
- Replace ListItem downcast unwrap with if-let
- Fix resize preview division by zero on degenerate images
- Clamp rename cursor position to prevent overflow panic
- Watch mode: skip output dirs to prevent infinite loop
- Watch mode: drop tx sender so channel closes on exit
- Watch mode: add delay for partially-written files
- Watch mode: warn and skip unmatched files instead of wrong preset
- Clean temp download directory on app close
- Replace action downcast unwrap with checked if-let
- Add BatchResult.output_files for accurate undo tracking

pixstrip-gtk/src/steps/step_resize.rs

@@ -445,12 +445,14 @@ pub fn build_resize_page(state: &AppState) -> adw::NavigationPage {
             std::thread::spawn(move || {
                 let result = (|| -> Option<Vec<u8>> {
                     let img = image::open(&path).ok()?;
-                    let target_w = if render_tw > 0 { render_tw } else { img.width() };
+                    let target_w = if render_tw > 0 { render_tw } else { img.width().max(1) };
                     let target_h = if render_th > 0 {
                         render_th
-                    } else {
+                    } else if img.width() > 0 {
                         let scale = target_w as f64 / img.width() as f64;
-                        (img.height() as f64 * scale).round() as u32
+                        (img.height() as f64 * scale).round().max(1.0) as u32
+                    } else {
+                        target_w
                     };
                     let resized = if mode == 0 && render_th > 0 {
                         // Exact: stretch to exact dimensions