fix(images): pick the decoder from the bytes, not the extension

`image::open` chooses the decoder by extension, so a PNG saved as .jpg
(Android screenshots, chat exports) failed thumbnails, previews,
perceptual hashes, insights and face crops with "Illegal start bytes:
8950". New `file_types::open_image` sniffs the header and falls back to
the extension when it's unknown; every decode site uses it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ai/insight_generator.rs

@@ -463,7 +463,7 @@ impl InsightGenerator {
         log::debug!("Loading image for vision model: {:?}", full_path);
 
         // Open and decode the image
-        let img = image::open(&full_path)
+        let img = crate::file_types::open_image(&full_path)
             .map_err(|e| anyhow::anyhow!("Failed to open image file: {}", e))?;
 
         let (original_width, original_height) = (img.width(), img.height());