perf(thumbs): switch thumbnail encoding from WebP to JPEG q=80

Replace all 3 ImageFormat::WebP encode sites with JpegEncoder q=80.
Update fast-path to detect JPEG SOI instead of RIFF/WEBP magic.
Change file extension .webp -> .jpg, Content-Type headers, and
browser toBlob. Remove unused ImageFormat import and stale comments.
The webp feature stays for DECODING uploaded WebP images.

static/js/features/library/photos.js

@@ -268,9 +268,9 @@ const photosView = {
             const ctx = canvas.getContext('2d');
             ctx.drawImage(video, 0, 0, canvas.width, canvas.height);
 
-            // Try WebP first, fall back to JPEG
-            const mimeType = typeof canvas.toBlob === 'function'
-                ? 'image/webp' : 'image/jpeg';
+            // JPEG: explicit quality control, universally supported,
+            // and server stores as-is when dimensions fit (zero re-encode).
+            const mimeType = 'image/jpeg';
 
             canvas.toBlob((blob) => {
                 if (!blob) {