feat: add config file with profiles support (#295)

## 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

```bash
# 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

```toml
# 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

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

Closes #293

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: sassman