fix: improve public download headers for PDF viewer app compatibility

- Add proper MIME type detection
- Set Content-Disposition: inline for viewing in apps
- Add Accept-Ranges header for byte range requests
- Add CORS header for cross-origin app access
- Support ?download=1 param for explicit download mode

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

drive_routes.py

@@ -1,7 +1,7 @@
 import os
 import shutil
 import gdown
-from flask import Blueprint, render_template, request, jsonify, current_app, send_from_directory, url_for, redirect, session
+from flask import Blueprint, render_template, request, jsonify, current_app, send_from_directory, send_file, url_for, redirect, session
 from flask_login import login_required, current_user
 from database import get_db_connection
 from datetime import datetime
@@ -590,7 +590,30 @@ def public_api_download(file_id):
             with open(file_path, 'wb') as f:
                 download_file_to_stream(service, file_id, f)
 
-        return send_from_directory(cache_dir, safe_name, as_attachment=False)
+        # Determine MIME type
+        import mimetypes
+        mime_type = mimetypes.guess_type(safe_name)[0] or 'application/octet-stream'
+
+        # Check if download mode requested
+        download_mode = request.args.get('download', '').lower() in ('1', 'true', 'yes')
+
+        # Return file with proper headers for both inline viewing and download
+        response = send_file(
+            file_path,
+            mimetype=mime_type,
+            as_attachment=download_mode,
+            download_name=filename
+        )
+
+        # Add headers for better compatibility with PDF viewer apps
+        if not download_mode:
+            response.headers['Content-Disposition'] = f'inline; filename="{filename}"'
+
+        # Allow cross-origin access for apps
+        response.headers['Access-Control-Allow-Origin'] = '*'
+        response.headers['Accept-Ranges'] = 'bytes'
+
+        return response
     except Exception as e:
         return f"Error: {e}", 500