Clean up low-severity code quality issues

- Share IMAGE_EXTENSIONS between discovery.rs and watcher.rs (DRY)
- Extract compute_renamed_path() to deduplicate ~100 lines in executor
- Extract estimate_text_dimensions() to deduplicate watermark calc (3 copies)
- Fix encoder fallback defaults: WebP 85, AVIF 63 (match QualityPreset::High)
- Extract watch_config_dir() and load_watches() helpers in CLI (4 copies)
- Remove redundant else branches after unwrap_or_default()
- Rename misleading chrono_timestamp() to unix_timestamp()

pixstrip-core/src/encoder.rs

@@ -40,8 +40,8 @@ impl OutputEncoder {
         match format {
             ImageFormat::Jpeg => self.encode_jpeg(img, quality.unwrap_or(85)),
             ImageFormat::Png => self.encode_png(img, quality.unwrap_or(3)),
-            ImageFormat::WebP => self.encode_webp(img, quality.unwrap_or(80)),
-            ImageFormat::Avif => self.encode_avif(img, quality.unwrap_or(80)),
+            ImageFormat::WebP => self.encode_webp(img, quality.unwrap_or(85)),
+            ImageFormat::Avif => self.encode_avif(img, quality.unwrap_or(63)),
             ImageFormat::Gif => self.encode_fallback(img, image::ImageFormat::Gif),
             ImageFormat::Tiff => self.encode_fallback(img, image::ImageFormat::Tiff),
             ImageFormat::Bmp => self.encode_fallback(img, image::ImageFormat::Bmp),