Fix pipeline order, add selective metadata stripping, rename case/regex

- Move watermark step after compress in processing pipeline to match
  design doc order (resize, adjustments, convert, compress, metadata,
  watermark, rename)
- Implement selective EXIF metadata stripping for Privacy and Custom
  modes using little_exif tag filtering (GPS, camera, software,
  timestamps, copyright categories)
- Add case conversion support to rename (none/lower/upper/title)
- Add regex find-and-replace on original filenames
- Wire case and regex controls in rename step UI to JobConfig
- Add regex crate dependency to pixstrip-core

pixstrip-core/src/preset.rs

@@ -120,6 +120,9 @@ impl Preset {
                 counter_start: 1,
                 counter_padding: 3,
                 template: None,
+                case_mode: 0,
+                regex_find: String::new(),
+                regex_replace: String::new(),
             }),
         }
     }
@@ -177,6 +180,9 @@ impl Preset {
                 counter_start: 1,
                 counter_padding: 4,
                 template: Some("{exif_date}_{name}_{counter:4}".into()),
+                case_mode: 0,
+                regex_find: String::new(),
+                regex_replace: String::new(),
             }),
         }
     }