fix: ICO needs CLI decode, AVIF compress missing options, remove JXL output

- ICO: Sharp cannot decode ICO files. Added ImageMagick-based ICO decoder
  that extracts the largest embedded image. Added ICO to CLI_DECODED_FORMATS
  and SERVER_PREVIEW_EXTENSIONS. Removed from BROWSER_PREVIEWABLE sets.
- AVIF compress: Sharp's AVIF encoder requires effort option. Added
  formatOpts() helper that supplies effort:4 for AVIF format.
- JXL output: Docker's bundled libvips lacks the JXL encoder plugin.
  Removed JXL as a convert output target to avoid guaranteed failures.
  JXL remains fully supported as an input format.

apps/web/src/lib/image-preview.ts

@@ -3,6 +3,7 @@ import { formatHeaders } from "@/lib/api";
 const SERVER_PREVIEW_EXTENSIONS = new Set([
   "heic", "heif", "hif",                       // HEIF
   "jxl",                                         // JPEG XL (Chrome dropped support)
+  "ico",                                         // ICO (Sharp can't decode)
   "dng", "cr2", "nef", "arw", "orf", "rw2",     // Camera RAW
   "tga",                                         // Targa
   "psd",                                         // Photoshop