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

My original rationale was that LLMs would be inflating the token count, but it's also 'cleaner' from a code perspective, and makes reviews easier.

- readable
- spec language

```diff
+// SPDX-License-Identifier: GPL-3.0-or-later
+// Copyright (c) 2025 David Allison
-/*
- * Copyright (c) 2025 David Allison
- *
- * This program is free software; you can redistribute it and/or modify it under
- * the terms of the GNU General Public License as published by the Free Software
- * Foundation; either version 3 of the License, or (at your option) any later
- * version.
- *
- * This program is distributed in the hope that it will be useful, but WITHOUT ANY
- * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
- * PARTICULAR PURPOSE. See the GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License along with
- * this program. If not, see .
- */
```

EDIT: copyright is now optional - https://github.com/ankidroid/Anki-Android/blob/main/docs/contributing/copyright-headers.md

```diff
+ // SPDX-License-Identifier: GPL-3.0-or-later
- // Copyright (c) 2025 David Allison
```

This issue will be completed when all mono-licensed files use SPDX.

----

**Future extensions**

* Handle multi-licensed files
* Mono-licensed files are the majority of the codebase. We are time-limited, and deviations can be handled on a case-by-case basis
* This issue exists to streamline the code submission process, not as a full SPDX implementation
- [x] Make copyright headers optional