feat: add config file with profiles support

[sassman]

Summary

This PR adds support for a configuration file with named profiles, making it easy to save and reuse your favorite t-rec settings.

New Features

• Config file support: Save your default settings in ~/.config/t-rec/config.toml (or local t-rec.toml)
• Named profiles: Create different presets for various use cases (e.g., demo, quick, presentation)
• Easy setup: Run t-rec --init-config to create a starter config file
• Profile discovery: Use t-rec --list-profiles to see available profiles
• Home directory expansion: Use $HOME in wallpaper paths for portability

Usage Examples

# Create a starter config file
t-rec --init-config

# Use a named profile
t-rec --profile demo

# List available profiles
t-rec --list-profiles

Example Config File

# Default settings applied to all recordings
[default]
wallpaper = "ventura"
wallpaper-padding = 60

# Named profiles for different use cases
[profiles.demo]
wallpaper = "ventura"
wallpaper-padding = 100
start-pause = "5s"
idle-pause = "5s"

[profiles.quick]
quiet = true
idle-pause = "1s"

[profiles.custom]
wallpaper = "$HOME/Pictures/my-wallpaper.png"
wallpaper-padding = 80

Supported Settings

All CLI options can be set in the config file:

• wallpaper - Background wallpaper (built-in name or path)
• wallpaper-padding - Padding around the recording (1-500px)
• decor - Border decoration style (shadow or none)
• bg - Background color (white, black, transparent)
• quiet - Suppress the Ctrl+D banner
• verbose - Enable verbose output
• video / video-only - Generate MP4 video
• natural - Disable idle detection
• start-pause / end-pause / idle-pause - Timing controls
• output - Output file name

Priority Order

Settings are applied in this order (later values override earlier):

1. Built-in defaults
2. Config file [default] section
3. Config file [profiles.<name>] section (if --profile used)
4. CLI arguments

Test plan

• cargo build succeeds
• cargo test passes (17 tests)
• cargo clippy passes
• Manual test: t-rec --init-config creates config file
• Manual test: t-rec --list-profiles shows profiles
• Manual test: t-rec --profile demo uses profile settings
• Manual test: CLI args override profile settings

Closes #293