Add .properties config support and audio device selection

[monsterwendy]

Summary

This PR adds support for loading .properties config files — the format used by the C++ SDL frontend (projectMSDL) — so users can migrate from the C++ frontend without rewriting their config.

What's new

.properties config file support (src/properties.rs)

• Auto-detected by file extension when passed via -c config.properties
• Parses the key: value format used by projectMSDL
• Two-pass parsing: window.* keys first, then projectM.* / audio.* keys
• Handles Windows-style backslash paths (normalizes to forward slashes)
• Spaces in paths work without quoting

Property mappings:

.properties key	Setting
projectM.presetPath	--preset-path
projectM.shuffleEnabled	--shuffle-enabled (new)
projectM.transitionDuration	--transition-duration (new)
projectM.displayDuration	--preset-duration
audio.device	--audio-device (new — see below)
window.width/height/left/top	window geometry

Audio device selection (src/app/audio.rs)

• New --audio-device CLI flag / PM_AUDIO_INPUT env var
• Finds device by name at startup (case-insensitive, falls back to substring match)
• If not found, falls back to default device with a warning

Preset path fix

• After loading the user's preset path, immediately calls playlist_play_next() so presets from the configured folder start playing right away instead of waiting for the first transition timer

Window config (src/app.rs)

• Window size (width/height) now read from config

New CLI flags added to Settings:

• --audio-device / PM_AUDIO_INPUT
• --shuffle-enabled / PM_SHUFFLE
• --transition-duration / PM_TRANSITION_DURATION

README updates

• Added .properties format usage example
• Added macOS note: terminal app needs Microphone permission in System Settings → Privacy & Security, or audio capture will silently return no data

Example config

A Windows projectMSDL config file can be used directly:

audio.device: BlackHole 2ch
projectM.presetPath: /Users/delta/presets
projectM.shuffleEnabled: true
projectM.transitionDuration: 10
projectM.displayDuration: 60
window.width: 1280
window.height: 720

./projectm -c projectMSDL.properties

Test plan

• .properties file loads correctly (unit tests in src/properties.rs)
• cargo test passes
• Audio device selection works on macOS and Linux
• Preset path loads immediately on startup
• Existing .toml/.json/.yaml config still works