chore: relicense to AGPL-3.0-or-later and add SPDX headers

Replaces LICENSE (GPLv3 -> AGPLv3) and prepends
`SPDX-License-Identifier: AGPL-3.0-or-later` to every source file
across decnet/, decnet_web/, tests/, scripts/, and tools/.

Rationale: closes the GPLv3 ASP loophole so any party operating a
modified DECNET as a network service must offer their modified
source. Personal copyright (Samuel Paschuan) + inbound=outbound
contributions make a future unilateral relicense infeasible.

- LICENSE: full AGPL-3.0 text (gnu.org/licenses/agpl-3.0.txt)
- COPYRIGHT: project copyright notice
- tools/add_spdx_headers.py: idempotent header injector
  (shebang- and PEP 263-aware)

Touches 1565 source files (.py, .ts, .tsx, .js, .jsx, .css, .sh).
No behavior change; comments only.

decnet_web/src/components/ThemeLab/ThemeLab.css

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: AGPL-3.0-or-later */
 /* ThemeLab — lab-page-specific layout only.
  * Colours come from index.css tokens; no hex literals here. */
 

decnet_web/src/components/ThemeLab/ThemeLab.tsx

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
 import React, { useEffect, useState } from 'react';
 import ThemeToggle from './ThemeToggle';
 import './ThemeLab.css';

decnet_web/src/components/ThemeLab/ThemeToggle.tsx

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
 import React, { useEffect, useState } from 'react';
 
 /* Dev-scoped Dark/Light theme toggle for the theme lab.

decnet_web/src/components/ThemeLab/__tests__/ThemeLab.test.tsx

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
 import { describe, it, expect } from 'vitest';
 import { render, screen } from '@testing-library/react';
 import ThemeLab from '../ThemeLab';

decnet_web/src/components/ThemeLab/__tests__/ThemeToggle.test.tsx

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
 import { describe, it, expect, beforeEach } from 'vitest';
 import { render, screen } from '@testing-library/react';
 import userEvent from '@testing-library/user-event';