feat: full HEIF/HEIC support, content-aware resize performance fix, UI improvements

- Add bidirectional HEIF support: decode (input) and encode (output) via system heif-convert/heif-enc
- Add server-side WebP preview generation for non-browser-previewable formats (HEIC, TIFF)
- Fix content-aware resize failing on HEIF input (decode before passing to caire)
- Fix content-aware resize timeout on large images by downscaling to max 1200px and using JPEG intermediate
- Add HEIF as target format in convert tool
- Add loading spinner for HEIF preview decode in file store
- Fix file picker not accepting HEIF files (explicit .heic,.heif,.hif extensions)
- Extend frontend timeout for medium tools to 180s with 45s progress animation
- Redesign rotate controls with preset buttons and compact flip section
- Remove misleading savings percentage from convert tool

apps/web/src/components/common/dropzone.tsx

@@ -10,7 +10,15 @@ interface DropzoneProps {
   currentFiles?: File[];
 }
 
+// Browsers may not map .heic/.heif to image/* in file pickers.
+// Append explicit extensions so they are selectable.
+function expandAccept(accept?: string): string | undefined {
+  if (!accept?.includes("image/*")) return accept;
+  return `${accept},.heic,.heif,.hif`;
+}
+
 export function Dropzone({ onFiles, accept, multiple = true, currentFiles = [] }: DropzoneProps) {
+  const resolvedAccept = expandAccept(accept);
   const [isDragging, setIsDragging] = useState(false);
 
   const handleDrag = useCallback((e: DragEvent) => {
@@ -35,7 +43,7 @@ export function Dropzone({ onFiles, accept, multiple = true, currentFiles = [] }
     const input = document.createElement("input");
     input.type = "file";
     input.multiple = multiple;
-    if (accept) input.accept = accept;
+    if (resolvedAccept) input.accept = resolvedAccept;
     input.onchange = (e) => {
       const files = Array.from((e.target as HTMLInputElement).files || []);
       if (files.length > 0) onFiles?.(files);