Add licensing information and update README for KIBU

- Introduced additional terms under Section 7 of the GNU AGPL v3 for KIBU, including attribution, marking of modified versions, and trademark usage.
- Added MIT License file for clarity on licensing options.
- Updated README.md to reflect the new licensing terms, including detailed usage rights and obligations.
- Created REUSE.toml for SPDX compliance and license annotations.
- Added THIRD_PARTY_NOTICES.md to clarify third-party dependencies and their licenses.
- Updated various source files and scripts to include SPDX headers for copyright and licensing.
- Enhanced footer in layout to display licensing information and project link.
- Implemented a new license page to present the AGPL and additional terms clearly.

LICENSES/LicenseRef-KIBU-Additional-Terms.txt

@@ -0,0 +1,23 @@
+# Additional Terms under Section 7 of the GNU AGPL v3
+
+The following additional terms apply to KIBU, Copyright (C) 2026 Paul Schoeneck.
+They supplement the GNU Affero General Public License, version 3 only.
+
+1. Attribution (Section 7b): Any copy or modified version that provides
+   a user interface must display, in an "About" page or footer visible
+   to users, the notice:
+   "Based on KIBU by Paul Schoeneck – https://git.voidcube.de/pixo2000/KIBU".
+
+2. Marking of modified versions (Section 7c): Modified versions must be
+   clearly marked in a reasonable manner as different from the original
+   version. Notices required by Section 5a of the GNU AGPL v3, including
+   the relevant date, must also be preserved.
+
+3. Names and trademarks (Section 7e): No rights under trademark law are
+   granted by this license to use the name "KIBU", its logos, or other
+   identifying marks. Any use remains subject to applicable trademark
+   law, including its limitations and exceptions. This term does not
+   prohibit the attribution required above, truthful identification of
+   the origin of the work, or other uses permitted by applicable law.
+   There is no general requirement to rename every modified version
+   merely because it has been modified.