Add GPLv3 license and per-file copyright headers

License Flipper Blue++ under GPL-3.0-or-later, matching the Flipper Zero
firmware it builds on and links against. Add the verbatim GPLv3 text as
LICENSE, an SPDX + copyright header to every original source file and
build.sh, and a License section to the README noting the app sources are
(C) 2026 Andy Dixon while the upstream firmware/SDK remain (C) Flipper
Devices Inc., both GPLv3. Headers are comment-only; the full suite still
builds and all faps link.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LEaD4ZeAfvWLBUxXp8G11r

README.md

@@ -452,3 +452,20 @@ Each app directory holds its `application.fam`, `icon.png` and sources. To add
 an app, drop a directory next to these and add its name to `APPS=(...)` in
 `build.sh`; if it includes `ble_central.h`, `build.sh` stages `libble` into it
 automatically.
+
+---
+
+## License
+
+Flipper Blue++ is licensed under the **GNU General Public License v3.0 or
+later** (see [`LICENSE`](LICENSE)), matching the Flipper Zero firmware it builds
+on and links against. Every original source file carries an SPDX header.
+
+- The app sources (`libble/`, `bt_inspector/`, `ble_hid_host/`, `ble_sensor/`,
+  `ble_tracker/`, `ble_gattfuzz/`, `ble_beacon/`, `ble_gattsrv/`, `subghz_scan/`,
+  `subghz_spectrum/`, `battery_health/`) and `build.sh` are
+  © 2026 Andy Dixon <andy@dixon.cx>.
+- The Flipper Zero firmware and SDK that `build.sh` clones and builds are
+  © Flipper Devices Inc. and contributors, also under GPLv3. This repository
+  does not redistribute that source; `build.sh` fetches it from the upstream
+  repository at build time.