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

subghz_scan/subghz_scan.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// Copyright (C) 2026 Andy Dixon <andy@dixon.cx>
 // Sub-GHz Scanner: sweep RSSI across a frequency range in fixed steps, show the
 // strongest bins, and log any frequency whose RSSI crosses a threshold. Uses the
 // region-unlocked firmware so the whole hardware-tunable range is available.