feat: add multiple shell profiles support

[OleksandrKucherenko]

Closes #622

Summary

This PR adds support for multiple shell profiles in Rio terminal, allowing users to:

1. Define a default shell via [shell] (existing behavior)
2. Define additional shell profiles via [[shells]] array
3. Use Ctrl+Shift+T to create tabs with the default profile (unchanged)
4. Use Ctrl+Shift+Y to open a profile selector to choose which shell to use

Configuration Example

# Default shell
[shell]
program = "pwsh"
args = []

# Additional shell profiles
[[shells]]
name = "WSL"
program = "C:\\Windows\\System32\\wsl.exe"
args = []

[[shells]]
name = "CMD"
program = "C:\\Windows\\System32\\cmd.exe"
args = []

Profile Selector

When you press Ctrl+Shift+Y (or Cmd+Y on macOS), a selector overlay appears:

• Use j/k or arrow keys to navigate
• Press 1-9 to directly select a profile
• Press Enter to confirm selection
• Press Escape to cancel

Changes

• Extended Shell struct with optional name field
• Added shells: Vec<Shell> field to Config
• Added ShellProfileSelector action with keybinding
• Created ShellSelector state module for managing selector UI
• Added create_tab_with_shell() method for creating tabs with specific shells
• Created shell selector UI renderer
• Integrated keyboard navigation and rendering
• Updated documentation with examples

Testing

1. Add shell profiles to your config
2. Press Ctrl+Shift+Y to open selector
3. Navigate and select a profile
4. Verify a new tab opens with the selected shell

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