Add SPDX header

[AndyScherzinger]

WIP due to issues with compiled assets or their license info respectively

[AndyScherzinger]

@susnux could you maybe lend me a hand with the license file generation you did in server to also do in this app?

The issue, similar to server is, in this case:

The following files have no copyright and licensing information:
* js\files_pdfviewer-data_image_svg_xml_3csvg_20xmlns_27http_www_w3_org_2000_svg_27_20width_2724_27_20height_2724_-b2e651.js
* js\files_pdfviewer-main.js
* js\files_pdfviewer-main.js.map
* js\files_pdfviewer-node_modules_nextcloud_dialogs_dist_chunks_index-C8hdiGrE_mjs.js
* js\files_pdfviewer-node_modules_nextcloud_dialogs_dist_chunks_index-C8hdiGrE_mjs.js.map
* js\files_pdfviewer-public.js
* js\files_pdfviewer-public.js.map
* js\files_pdfviewer-vendors-node_modules_nextcloud_dialogs_dist_chunks_FilePicker-CaGLN3Vi_mjs.js
* js\files_pdfviewer-vendors-node_modules_nextcloud_dialogs_dist_chunks_FilePicker-CaGLN3Vi_mjs.js.map
* js\files_pdfviewer-workersrc.js
* js\files_pdfviewer-workersrc.js.map

The following files have no licensing information:
* js\files_pdfviewer-main.js.LICENSE.txt
* js\files_pdfviewer-vendors-node_modules_nextcloud_dialogs_dist_chunks_FilePicker-CaGLN3Vi_mjs.js.LICENSE.txt

[AndyScherzinger]

/compile /