docs: convert licenses to SPDX format

[david-allison]

[!INFO] Assisted-by: Claude Opus 4.8
proposed the recently modified files to look at

41 recently changed files.

AnkiDroid is standardizing licenses to SPDX.

I removed my copyright lines from the files as-per docs/contributing/copyright-headers.md

I added headers to files which were missing copyright.

• Issue Move to SPDX for file-level license headers (mono-license GPL/LGPL only) #20954

Checklist

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner