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/rename_tests.rs

@@ -1,4 +1,7 @@
-use pixstrip_core::operations::rename::{apply_template, resolve_collision};
+use pixstrip_core::operations::rename::{
+    apply_template, apply_regex_replace, apply_space_replacement,
+    apply_special_chars, apply_case_conversion, resolve_collision,
+};
 
 #[test]
 fn template_basic_variables() {
@@ -93,3 +96,185 @@ fn no_collision_returns_same() {
     let resolved = resolve_collision(&path);
     assert_eq!(resolved, path);
 }
+
+// --- Regex replace tests ---
+
+#[test]
+fn regex_replace_basic() {
+    let result = apply_regex_replace("hello_world", "_", "-");
+    assert_eq!(result, "hello-world");
+}
+
+#[test]
+fn regex_replace_pattern() {
+    let result = apply_regex_replace("IMG_20260307_001", r"\d{8}", "DATE");
+    assert_eq!(result, "IMG_DATE_001");
+}
+
+#[test]
+fn regex_replace_invalid_pattern_returns_original() {
+    let result = apply_regex_replace("hello", "[invalid", "x");
+    assert_eq!(result, "hello");
+}
+
+#[test]
+fn regex_replace_empty_find_returns_original() {
+    let result = apply_regex_replace("hello", "", "x");
+    assert_eq!(result, "hello");
+}
+
+// --- Space replacement tests ---
+
+#[test]
+fn space_replacement_none() {
+    assert_eq!(apply_space_replacement("hello world", 0), "hello world");
+}
+
+#[test]
+fn space_replacement_underscore() {
+    assert_eq!(apply_space_replacement("hello world", 1), "hello_world");
+}
+
+#[test]
+fn space_replacement_hyphen() {
+    assert_eq!(apply_space_replacement("hello world", 2), "hello-world");
+}
+
+#[test]
+fn space_replacement_dot() {
+    assert_eq!(apply_space_replacement("hello world", 3), "hello.world");
+}
+
+#[test]
+fn space_replacement_camelcase() {
+    assert_eq!(apply_space_replacement("hello world", 4), "helloWorld");
+}
+
+#[test]
+fn space_replacement_remove() {
+    assert_eq!(apply_space_replacement("hello world", 5), "helloworld");
+}
+
+// --- Special chars tests ---
+
+#[test]
+fn special_chars_keep_all() {
+    assert_eq!(apply_special_chars("file<name>.txt", 0), "file<name>.txt");
+}
+
+#[test]
+fn special_chars_filesystem_safe() {
+    assert_eq!(apply_special_chars("file<name>", 1), "filename");
+}
+
+#[test]
+fn special_chars_web_safe() {
+    assert_eq!(apply_special_chars("file name!@#", 2), "filename");
+}
+
+#[test]
+fn special_chars_alphanumeric_only() {
+    assert_eq!(apply_special_chars("file-name_123", 5), "filename123");
+}
+
+// --- Case conversion tests ---
+
+#[test]
+fn case_conversion_none() {
+    assert_eq!(apply_case_conversion("Hello World", 0), "Hello World");
+}
+
+#[test]
+fn case_conversion_lowercase() {
+    assert_eq!(apply_case_conversion("Hello World", 1), "hello world");
+}
+
+#[test]
+fn case_conversion_uppercase() {
+    assert_eq!(apply_case_conversion("Hello World", 2), "HELLO WORLD");
+}
+
+#[test]
+fn case_conversion_title_case() {
+    assert_eq!(apply_case_conversion("hello world", 3), "Hello World");
+}
+
+#[test]
+fn case_conversion_title_preserves_separators() {
+    assert_eq!(apply_case_conversion("hello-world_foo", 3), "Hello-World_Foo");
+}
+
+// --- RenameConfig::apply_simple edge cases ---
+
+use pixstrip_core::operations::RenameConfig;
+
+fn default_rename_config() -> RenameConfig {
+    RenameConfig {
+        prefix: String::new(),
+        suffix: String::new(),
+        counter_start: 1,
+        counter_padding: 3,
+        counter_enabled: false,
+        counter_position: 3,
+        template: None,
+        case_mode: 0,
+        replace_spaces: 0,
+        special_chars: 0,
+        regex_find: String::new(),
+        regex_replace: String::new(),
+    }
+}
+
+#[test]
+fn apply_simple_no_changes() {
+    let cfg = default_rename_config();
+    assert_eq!(cfg.apply_simple("photo", "jpg", 1), "photo.jpg");
+}
+
+#[test]
+fn apply_simple_with_prefix_suffix() {
+    let mut cfg = default_rename_config();
+    cfg.prefix = "web_".into();
+    cfg.suffix = "_final".into();
+    assert_eq!(cfg.apply_simple("photo", "jpg", 1), "web_photo_final.jpg");
+}
+
+#[test]
+fn apply_simple_counter_after_suffix() {
+    let mut cfg = default_rename_config();
+    cfg.counter_enabled = true;
+    cfg.counter_position = 3;
+    assert_eq!(cfg.apply_simple("photo", "jpg", 1), "photo_001.jpg");
+}
+
+#[test]
+fn apply_simple_counter_replaces_name() {
+    let mut cfg = default_rename_config();
+    cfg.counter_enabled = true;
+    cfg.counter_position = 4;
+    assert_eq!(cfg.apply_simple("photo", "jpg", 1), "001.jpg");
+}
+
+#[test]
+fn apply_simple_empty_name() {
+    let cfg = default_rename_config();
+    assert_eq!(cfg.apply_simple("", "png", 1), ".png");
+}
+
+#[test]
+fn apply_simple_case_lowercase() {
+    let mut cfg = default_rename_config();
+    cfg.case_mode = 1;
+    assert_eq!(cfg.apply_simple("MyPhoto", "JPG", 1), "myphoto.JPG");
+}
+
+#[test]
+fn apply_simple_large_counter_padding_capped() {
+    let mut cfg = default_rename_config();
+    cfg.counter_enabled = true;
+    cfg.counter_padding = 100; // should be capped to 10
+    cfg.counter_position = 3;
+    let result = cfg.apply_simple("photo", "jpg", 1);
+    // Counter portion should be at most 10 digits
+    assert!(result.len() <= 22); // "photo_" + 10 digits + ".jpg"
+}