✨ Add comprehensive FEATURES.md documenting all divergences from upstream

[Copilot]

Overview

This PR adds a comprehensive FEATURES.md document that catalogs all features and major changes in this fork that diverge from the original afaden/babelpod repository.

What's Added

📄 FEATURES.md Document

A complete feature list documenting:

1. 11 Major Features added to this fork:

   • 🎧 Bluetooth Input Support (cherry-picked from DerDaku)
   • 🔄 Automatic Input Recovery for USB device disconnects
   • 🌐 Enhanced mDNS Service Discovery with dnssd2 (from DerDaku)
   • ⚙️ Optional PCM Device Scanning with environment variables (from DerDaku)
   • 🛡️ Comprehensive Error Handling & Reliability improvements
   • 🎚️ UI Bug Fixes (volume slider, output selection)
   • 🔊 Improved AirPlay Streaming Stability (from DerDaku)
   • 📦 Updated Dependencies including airtunes2 v2.4.9 (from DerDaku)
   • 🎮 Systemd Service Support for auto-start on boot
   • 🧪 Testing Infrastructure with Jest
   • 📚 Enhanced Documentation

2. Clear Attribution for changes cherry-picked from DerDaku/babelpod:

   • Lists exactly what was merged from DerDaku's fork
   • Explains what was NOT merged and why
   • Credits DerDaku's contributions prominently

3. Important Design Choices with reasoning:

   • PCM scanning default behavior (backward compatibility)
   • Debouncing timeout values (balancing UX and performance)
   • Automatic retry logic (reliability without excessive delays)
   • Error message auto-dismiss timing
   • Dependency pinning strategy

4. Requirement Updates:

   • New optional dependencies (bluez for Bluetooth)
   • Environment variable configuration (DISABLE_PCM, BABEL_PORT)
   • Node.js compatibility notes
   • Raspberry Pi Zero/Zero 2 W specific guidance

5. Visual Appeal with emoji throughout for easy scanning and engagement

Why This Matters

This document serves as:

• A feature comparison between this fork and upstream
• Attribution documentation for community contributions
• A technical reference for understanding design decisions
• Migration guidance for users coming from upstream

Structure

The document is organized into clear sections:

• Overview and summary
• Detailed feature descriptions with technical details
• Design choices and rationale
• Requirement updates
• Changes from DerDaku (what was and wasn't merged)
• Breaking changes (none - all backward compatible)
• Future work roadmap

No Code Changes

This PR is documentation-only and makes no changes to the codebase. As requested in the issue, no Pull Request merge is needed - this serves as historical documentation of the fork's divergence from upstream.

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Fixes #[issue_number] (List features added since last merge from upstream)

Original prompt

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This section details on the original issue you should resolve

<issue_title>List features added since last merge from upstream</issue_title>
<issue_description>1. List all features and major changes introduced that diverge from afaden/babelpod since forking/merging from upstream.
2. Note changes cherry-picked from DerDaku/babelpod.
3. Note any important choices, changes, or requirement updates and some of the reasons for them.
4. Use Emoji.

No Pull Request is needed for this issue.</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #8

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