feat: document scanner — scan file uploads for PPI before sending

New module: document-scanner.js
- PDF: extracts text from content streams, scans for PPI, converts to
  sanitized plaintext for upload (PDFs can't be reliably edited in-place)
- DOCX/XLSX: extracts text from XML entries, scans for PPI
- Text files: direct string substitution
- Handles scanned/image-only PDFs gracefully (skips with message)

Fetch interceptor (content.js):
- Now intercepts FormData uploads (file uploads) in addition to JSON
- Processes each file through document scanner
- Preview mode for PDF/DOCX/XLSX: shows PPI findings with counts,
  user clicks "Substitute & Upload" or "Upload Original"
- Text files substituted silently (no preview friction)
- String form fields also substituted
- 30-second auto-dismiss on preview (uploads original if no response)

Preview UI (content.css):
- Centered overlay with dark theme matching existing warning UI
- Shows original → substituted pairs for each PPI found
- File format note (e.g. "PDF will be converted to plain text")

https://claude.ai/code/session_01SWSwDfMVij53bCTNSCLMwn

src/content/content.css

@@ -244,3 +244,113 @@
   opacity: 1;
   transform: translateY(0);
 }
+
+/* Document scan preview overlay */
+.ss-doc-preview {
+  position: fixed;
+  top: 50%;
+  left: 50%;
+  transform: translate(-50%, -50%) scale(0.95);
+  max-width: 480px;
+  width: 90vw;
+  background: #1a1a1a;
+  color: #e5e7eb;
+  border: 1px solid #f59e0b;
+  border-radius: 12px;
+  padding: 16px 20px;
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
+  font-size: 12px;
+  z-index: 9999999;
+  box-shadow: 0 8px 32px rgba(0, 0, 0, 0.6);
+  opacity: 0;
+  pointer-events: none;
+  transition: opacity 0.2s, transform 0.2s;
+  max-height: 70vh;
+  overflow-y: auto;
+}
+
+.ss-doc-preview.visible {
+  opacity: 1;
+  transform: translate(-50%, -50%) scale(1);
+  pointer-events: auto;
+}
+
+.ss-dp-header {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  margin-bottom: 8px;
+}
+
+.ss-dp-count {
+  font-size: 11px;
+  color: #f59e0b;
+  font-weight: 600;
+}
+
+.ss-dp-note {
+  font-size: 11px;
+  color: #9ca3af;
+  margin-bottom: 10px;
+  font-style: italic;
+}
+
+.ss-dp-items {
+  max-height: 200px;
+  overflow-y: auto;
+  margin-bottom: 12px;
+}
+
+.ss-dp-item {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  padding: 4px 0;
+  border-bottom: 1px solid #333;
+  font-size: 11px;
+}
+
+.ss-dp-item:last-child { border-bottom: none; }
+
+.ss-dp-orig {
+  color: #f87171;
+  background: #1f1f1f;
+  padding: 2px 5px;
+  border-radius: 3px;
+}
+
+.ss-dp-repl {
+  color: #4ade80;
+  background: #1f1f1f;
+  padding: 2px 5px;
+  border-radius: 3px;
+}
+
+.ss-dp-actions {
+  display: flex;
+  gap: 8px;
+}
+
+.ss-dp-btn {
+  flex: 1;
+  padding: 8px 12px;
+  border: none;
+  border-radius: 6px;
+  font-size: 12px;
+  font-weight: 500;
+  cursor: pointer;
+}
+
+.ss-dp-confirm {
+  background: #10b981;
+  color: #fff;
+}
+
+.ss-dp-confirm:hover { background: #059669; }
+
+.ss-dp-cancel {
+  background: #374151;
+  color: #e5e7eb;
+}
+
+.ss-dp-cancel:hover { background: #4b5563; }