docs: add NOTICE, TRADEMARK, and README forking guidance (#474)

* docs: add NOTICE and TRADEMARK files

Adds a NOTICE.md with the GPLv3 §7(b) attribution-preservation terms
that derivative works must carry forward, and a TRADEMARK.md that
covers the Psysonic name, logo, brand identity, and the names of
original Psysonic features (in particular Orbit) — none of which are
licensed under the GPLv3.

Forks remain free under GPLv3 to reuse the code, but must rename
Psysonic-branded assets and feature names, and must preserve the
attribution that original Psysonic features were designed and
implemented in this project.

* docs(readme): add Forks and Attribution section

Points readers at the GPLv3 freedoms while clarifying the expectations
for forks: keep the project name and feature names as documented in
TRADEMARK.md, preserve attribution as required by NOTICE.md, and do
not present original Psysonic work (such as Orbit) as independent
creations of a fork.

NOTICE.md

@@ -0,0 +1,40 @@
+# Notice
+
+Psysonic is licensed under the GNU GPLv3.
+
+Psysonic, including features such as **Orbit** (synchronized shared
+listening sessions), was originally designed and implemented by the
+Psysonic project maintainers and contributors.
+
+Forks and modified versions are welcome under the GPLv3, but they
+must preserve copyright notices, license information, and clear
+attribution to the original Psysonic project.
+
+Modified versions must not imply that original Psysonic features were
+independently created by the fork, nor imply endorsement by the
+Psysonic project.
+
+## Required Attribution for Derivative Works
+
+The following attribution requirements apply to all derivative works
+under the additional terms permitted by **GPLv3 §7(b)**. Forks and
+redistributions must preserve these notices intact:
+
+1. The unmodified contents of this `NOTICE.md` file must be retained
+   in the source tree of any fork or derivative work.
+2. Derivative works that include or are based on the **Orbit**
+   feature — its design, protocol, or implementation — must clearly
+   state in their own documentation and release notes that the Orbit
+   feature was originally designed and implemented in Psysonic, and
+   must not present Orbit's design or implementation as independent
+   original work of the fork.
+3. The user-facing name "Orbit" is a Psysonic feature mark and may
+   not be reused by forks; see `TRADEMARK.md`. The attribution
+   requirement in (2) applies regardless of what the feature is
+   renamed to in a fork.
+4. The same applies to any other original Psysonic feature that a
+   fork chooses to retain.
+
+These terms do not restrict the freedoms granted by the GPLv3; they
+are attribution-preservation terms explicitly permitted by GPLv3
+§7(b) and must be carried forward in further redistribution.

README.md

@@ -185,6 +185,16 @@ GNU GPL v3.0
 
 ---
 
+## Forks and Attribution
+
+Psysonic is GPLv3 software. You are free to fork, modify, and redistribute it under the terms of that license.
+
+If you distribute a modified or rebranded version, please clearly state that it is based on Psysonic and preserve attribution to the original project.
+
+Features such as Orbit were originally designed and implemented in Psysonic. Forks must not present original Psysonic work as their own independent creation.
+
+---
+
 <div align="center">
 
 ## Stop using boring music clients.

TRADEMARK.md

@@ -0,0 +1,66 @@
+# Trademark Notice
+
+The Psysonic source code is licensed under the GNU GPLv3.
+The **Psysonic name, logo, wordmark, visual brand identity, and the
+names of original Psysonic features are not** — they are excluded
+from the GPLv3 grant and remain the property of the Psysonic project
+maintainers.
+
+## Covered Marks
+
+- The name **"Psysonic"** (in any capitalization or stylization)
+- The Psysonic logo and icon (`public/psysonic-inapp-logo.svg`,
+  app icons, tray icons, installer artwork)
+- The Psysonic wordmark and any derivative branding
+- The names of original Psysonic features that function as feature
+  identity, in particular **"Orbit"**
+
+## What You May Do
+
+- Use the Psysonic name to factually refer to the upstream project
+  (e.g. *"based on Psysonic"*, *"a fork of Psysonic"*, *"compatible
+  with Psysonic"*).
+- Redistribute unmodified Psysonic builds under the Psysonic name,
+  in accordance with the GPLv3.
+- Link to the official Psysonic repository, website, and community
+  channels.
+
+## What You May Not Do
+
+- Use the Psysonic name, logo, or branding for a **modified or
+  rebranded fork**, distribution, or derivative product.
+- Name a fork in a way that suggests it *is* Psysonic, an official
+  release of Psysonic, or endorsed by the Psysonic project (e.g.
+  "Psysonic Pro", "Psysonic Plus", "Psysonic Next", "MyPsysonic").
+- Reuse the Psysonic logo, icon set, or installer artwork in a fork
+  or derivative product, even with modifications.
+- Continue to use the feature name **"Orbit"** in a fork or
+  derivative. If your fork ships the Orbit feature (or code derived
+  from it), you must rename the user-facing feature.
+- Present original Psysonic features (such as Orbit) as independent
+  creations of a fork. See `NOTICE.md` for the attribution
+  requirements that apply to derivative works under GPLv3 §7(b).
+- Imply sponsorship, endorsement, or affiliation with the Psysonic
+  project where none exists.
+
+## Forking Done Right
+
+You are warmly invited to fork the code under the GPLv3.
+If you publish a fork or derivative, please:
+
+1. Choose your own distinct project name and branding.
+2. Keep the GPLv3 license, copyright notices, and the `NOTICE.md`
+   file intact (this is required, not optional — see `NOTICE.md`).
+3. State clearly that your project is *based on Psysonic* and link
+   back to the upstream repository.
+4. Replace Psysonic-branded assets (logos, icons, installer art,
+   wordmarks) with your own.
+5. If you ship features that originated in Psysonic (such as Orbit),
+   rename the user-facing feature and credit Psysonic as the
+   original designer and implementer.
+
+## Questions
+
+If you are unsure whether a planned use of the Psysonic name or
+branding is acceptable, please open an issue or contact the
+maintainers before publishing.