feat: multi-page PDF support and real-time redaction preview

[Copilot]

Implements two items from the project's planned future scope: processing all pages of a PDF during OCR/detection, and eliminating canvas flicker in the review modal when toggling entity masks.

Multi-page PDF support

SecureUploader.tsx — replaces the single-page pdfDimRef with a per-page pdfDimsRef: Record<number, {width, height}> and iterates all PDF pages:

for (let pageIdx = 0; pageIdx < pageCount; pageIdx++) {
    const { imageBuffer, width, height, canvas: pageCanvas } =
        await renderPDFPageToImage(parsed.buffer, pageIdx);
    pdfDimsRef.current[pageIdx] = { width, height };
    // Use the first page canvas as the preview image shown in the review modal
    if (pageIdx === 0) imageDataUrl = pageCanvas.toDataURL();
    const pageResult = await pool.runOCR(imageBuffer, 'image/png', pageIdx);
    words.push(...pageResult.words);
}

pdfProcessor.ts — redactPDF now accepts pageDimensions: Record<number, {width, height}> instead of a single imageWidth/imageHeight, scaling each entity's bounding box to its own page's coordinate space.

layer1.ts — mergeWordBBoxes derives pageIndex from the matched words' bounding boxes rather than a fixed parameter, ensuring entities are attributed to the correct page.

Real-time redaction preview

ReviewModal.tsx — splits the single effect that reloaded the image on every entity toggle into two:

• Image load (useEffect on imageDataUrl): renders image to canvas once, stores HTMLImageElement in imgRef
• Overlay redraw (useEffect on localEntities/scale): redraws only bounding boxes over the cached image

This eliminates the visible flicker when toggling individual entity masks.

Pre-existing build fixes

The project had 8 TypeScript compilation errors. Resolved all to bring the build to a passing state: missing PIIType entries in TYPE_LABELS/TYPE_COLORS, missing message field on OCRResponse, missing canvas property in pdfjs-dist RenderParameters, unused imports, and a PSM type mismatch in the OCR worker.

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