fix: block PDF uploads, fix PBM/PGM/PPM batch failures, add tests

- Added isImageFile() filter to all drop handlers (dropzone, collage,
  file-upload-area) so PDFs and non-image files are rejected on drop.
  Previously only the file picker's accept attribute filtered; drag-and-
  drop accepted anything.

- Added ppm, pgm, pbm to CLI_DECODED_FORMATS in file-validation.ts.
  These formats were missing, causing Sharp metadata checks to fail for
  some files during batch validation, which silently dropped them from
  results ("File not found in batch results").

- Added integration tests for PBM, PGM, PPM, TIFF, QOI, JP2, SVGZ
  single-file processing, plus a test confirming PDF is rejected.

apps/web/src/components/tools/collage-preview.tsx

@@ -23,6 +23,7 @@ import {
   X,
 } from "lucide-react";
 import { type DragEvent, useCallback, useEffect, useRef, useState } from "react";
+import { isImageFile } from "@/components/common/dropzone";
 import { type CollageTemplate, getTemplateById } from "@/lib/collage-templates";
 import { cn } from "@/lib/utils";
 import type { CellTransform, CollageImage } from "@/stores/collage-store";
@@ -95,7 +96,7 @@ function UploadArea() {
       e.preventDefault();
       e.stopPropagation();
       setIsDragging(false);
-      const files = Array.from(e.dataTransfer.files);
+      const files = Array.from(e.dataTransfer.files).filter(isImageFile);
       if (files.length > 0) addImages(files);
     },
     [addImages],