Keep animated images animated in the optimizer

- Animated WebP (J-82) was being flattened to its first frame: browsers
  have no animated WebP encoder, so the modal now sniffs the file header
  (4 KB range request), explains the limitation and disables Process
  instead of overwriting the file with a single frame.
- APNGs saved as .png took the still-image path and lost their frames;
  the header sniff looks for the acTL chunk, routes them to the animation
  pipeline (all frames + delays) and switches the UI to the animation
  options. The worker double-checks the header too, so no path can
  flatten an APNG.
- New dependency-free imageformat module, verified against real files
  (J-82 animated, static WebP/PNG, and a generated APNG named .png).

frontend/src/features/optimize/OptimizeModal.tsx

@@ -10,6 +10,7 @@ import type { MediaItem } from "@/lib/types";
 
 import { estimateVideoBitrate, VIDEO_CODEC_CANDIDATES } from "./bitrate";
 import { browserLabel, probeVideoEncoders, workingCodecs } from "./detect";
+import { sniffImageKind } from "./imageformat";
 import { deleteStoredFile } from "./opfs";
 import {
   DEFAULT_OPTIONS,
@@ -167,11 +168,33 @@ export function OptimizeModal({
   onClose: () => void;
 }) {
   const queryClient = useQueryClient();
-  const kind: OptimizeKind = kindForExtension(item.extension) ?? "image";
+  const extensionKind: OptimizeKind =
+    kindForExtension(item.extension) ?? "image";
   const [options, setOptions] = useState<OptimizeOptions>({
     ...DEFAULT_OPTIONS,
     maxDimension: null,
   });
+  // Sniff the header (4 KB range request) so APNGs saved as .png get the
+  // animation options and animated WebP is caught before a wasted run.
+  const sniffQuery = useQuery({
+    queryKey: ["image-kind", item.j_id, item.updated_at],
+    queryFn: async () => {
+      const response = await fetch(item.raw_url, {
+        headers: { Range: "bytes=0-4095" },
+      });
+      if (!response.ok && response.status !== 206) {
+        throw new Error(`HTTP ${response.status}`);
+      }
+      return sniffImageKind(await response.blob());
+    },
+    enabled: extensionKind === "image",
+    staleTime: Infinity,
+    retry: 0,
+  });
+  const sniffed = sniffQuery.data ?? null;
+  const kind: OptimizeKind = sniffed === "apng" ? "animation" : extensionKind;
+  const animatedWebp = sniffed === "animated-webp";
+  const apngAsPng = sniffed === "apng" && item.extension !== "apng";
   const { status, progress, stage, etaMs, error, result, run, reset } =
     useOptimizer();
   const [uploading, setUploading] = useState(false);
@@ -354,6 +377,21 @@ export function OptimizeModal({
               />
             </div>
 
+            {animatedWebp ? (
+              <p className="rounded-md border border-ctp-red/40 bg-ctp-red/10 px-3 py-2 text-xs text-ctp-red">
+                This is an animated WebP. Browsers have no animated WebP
+                encoder, so re-encoding would keep only the first frame —
+                nothing was changed.
+              </p>
+            ) : null}
+
+            {apngAsPng ? (
+              <p className="rounded-md border border-ctp-blue/40 bg-ctp-blue/10 px-3 py-2 text-xs text-ctp-blue">
+                Animated PNG detected — the animation options (scale, frame
+                rate, palette) apply.
+              </p>
+            ) : null}
+
             {truncated ? (
               <p className="rounded-md border border-ctp-red/40 bg-ctp-red/10 px-3 py-2 text-xs text-ctp-red">
                 The processed video is shorter than the original (
@@ -733,7 +771,7 @@ export function OptimizeModal({
             >
               Reset
             </Button>
-            <Button onClick={process} disabled={status === "running"}>
+            <Button onClick={process} disabled={status === "running" || animatedWebp}>
               {status === "done" ? "Process again" : "Process"}
             </Button>
             <Button

frontend/src/features/optimize/image.ts

@@ -11,6 +11,8 @@ import webpSimdWasmUrl from "@jsquash/webp/codec/enc/webp_enc_simd.wasm?url";
 import webpWasmUrl from "@jsquash/webp/codec/enc/webp_enc.wasm?url";
 import { threads } from "wasm-feature-detect";
 
+import { optimizeAnimation } from "./animation";
+import { sniffImageKind } from "./imageformat";
 import {
   extensionOf,
   type OptimizeOptions,
@@ -53,6 +55,18 @@ export async function optimizeImage(
 ): Promise<OptimizeResult> {
   const extension = extensionOf(filename);
   report(0.05, "Decoding the image…");
+  const sniffed = await sniffImageKind(blob);
+  if (sniffed === "apng") {
+    // An APNG usually has a .png extension; re-encoding it as a still image
+    // would drop every frame after the first.
+    return optimizeAnimation(blob, filename, options, report);
+  }
+  if (sniffed === "animated-webp") {
+    throw new Error(
+      "Animated WebP cannot be re-encoded in the browser (no animated WebP " +
+        "encoder is available) — this file was left untouched.",
+    );
+  }
   const bitmap = await createImageBitmap(blob);
 
   const sourceWidth = bitmap.width;

frontend/src/features/optimize/imageformat.ts

@@ -0,0 +1,62 @@
+/**
+ * Header sniffing for image files, so animated formats are not silently
+ * flattened to a single frame (an APNG is usually just a `.png`, and the
+ * browser decode path only sees the first frame either way).
+ *
+ * Dependency-free so it can run in workers and in Node-based checks.
+ */
+
+export type ImageFormatKind = "apng" | "animated-webp" | "static";
+
+const PNG_SIGNATURE = [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a];
+const HEAD_BYTES = 4096;
+
+function ascii(bytes: Uint8Array, start: number, end: number): string {
+  return String.fromCharCode(...bytes.slice(start, end));
+}
+
+/** APNG files carry an `acTL` chunk before the first `IDAT`. */
+function isApng(head: Uint8Array): boolean {
+  let offset = 8;
+  while (offset + 8 <= head.length) {
+    const length =
+      ((head[offset] << 24) |
+        (head[offset + 1] << 16) |
+        (head[offset + 2] << 8) |
+        head[offset + 3]) >>>
+      0;
+    const type = ascii(head, offset + 4, offset + 8);
+    if (type === "acTL") return true;
+    if (type === "IDAT") return false;
+    offset += 12 + length;
+    if (offset <= 0) return false;
+  }
+  return false;
+}
+
+/** WebP is animated when the VP8X chunk has the animation flag (bit 1). */
+function isAnimatedWebp(head: Uint8Array): boolean {
+  if (head.length < 21) return false;
+  if (ascii(head, 0, 4) !== "RIFF" || ascii(head, 8, 12) !== "WEBP") {
+    return false;
+  }
+  if (ascii(head, 12, 16) !== "VP8X") return false;
+  return (head[20] & 0x02) !== 0;
+}
+
+export function sniffImageBytes(bytes: Uint8Array): ImageFormatKind {
+  if (
+    bytes.length >= 8 &&
+    PNG_SIGNATURE.every((byte, index) => bytes[index] === byte)
+  ) {
+    return isApng(bytes) ? "apng" : "static";
+  }
+  return isAnimatedWebp(bytes) ? "animated-webp" : "static";
+}
+
+export async function sniffImageKind(file: Blob): Promise<ImageFormatKind> {
+  const head = new Uint8Array(
+    await file.slice(0, HEAD_BYTES).arrayBuffer(),
+  );
+  return sniffImageBytes(head);
+}