Add CHANGELOG.md tracking changes in this fork

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

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+# Changelog
+
+All notable changes to this fork of the OpenGD77 firmware (MD-UV380 / DM-1701).
+
+This project is still in pre-release iteration, so changes are collected as a
+single running list until the first versioned release.
+
+## Unreleased
+
+### Features
+
+- **Screen dimming** — optional idle dimming of the backlight to a fraction of
+  the configured brightness (Off / 100% / 50% / 30%), in Auto backlight mode.
+  Any activity (keys, rotary, RX, TX) restores full brightness. New setting in
+  Display Options.
+- **Mute** — one-tap audio mute:
+  - **SK2 short-tap** toggles mute on the VFO/Channel screens (except while a DMR
+    QSO is on screen, where SK2 keeps its "set TX TG to incoming" function).
+  - Turning the **volume knob fully down** is also treated as muted.
+  - While muted: a bold **"M"** appears in the status bar (left of the battery)
+    and the **red LED flashes** (0.5 s every 2 s).
+
+### Display / UI
+
+- **S-meter** redrawn as discrete blocks — 9 green blocks (S1–S9) plus 6 red
+  blocks (S9+10…+60), with gaps and a proportionally-filled leading block, and a
+  scale label row beneath (S1 S3 S5 S7 S9 / +20 +40 +60).
+- **7-segment "LED" frequency font** for the VFO and SK1-held frequency display,
+  with faintly-lit inactive segments; digits are wider than the normal font.
+- **Main-menu icons** — a small glyph next to each top-level menu entry.
+- Layout spacing adjusted so the contact/TG line, the DMR talkgroup/talker and
+  the private-call caller ID clear the new S-meter label row.
+
+### Fixes
+
+- **DMR no longer hard-faults.** The AMBE codec is now called through C function
+  pointers instead of the old inline assembler (which modern GCC miscompiled),
+  fixing the freeze-on-DMR that required a battery pull.
+- Fixed the DMR talkgroup/talker and private-call caller ID overlapping the
+  S-meter labels.
+
+### Build / Tooling
+
+- **Dockerized build** using the Arm GNU Toolchain 14 (matching the official
+  builds); always a clean build. `./flash.sh` helper for flashing with model,
+  language and DMR donor.
+- Integrated the OpenGD77 **screen grabber** (`gd-77_screen_grabber.py`) and the
+  GD-77 loader into the repo.
+- **`TOOLS.md`** documents building, flashing, screenshots and language files.
+
+### Misc
+
+- Added **Marcus DK1DA** to the firmware-info credits ("Modified Fork by").