feat(tui): add cortex-tui-components crate for standardized UI components

[echobt]

Summary

Create a new cortex-tui-components crate that provides reusable, high-level TUI components built on top of cortex-core's theme system.

Motivation

The existing TUI code has 100+ direct ratatui imports with redundant patterns for dropdowns, scrolling lists, borders, and input handling. This library consolidates those patterns into standardized, reusable components.

Key Features

Component Trait System

Unified interface for all interactive components:

pub trait Component {
    type Output;
    fn render(&self, area: Rect, buf: &mut Buffer);
    fn handle_key(&mut self, key: KeyEvent) -> ComponentResult<Self::Output>;
    fn focus_state(&self) -> FocusState;
    fn key_hints(&self) -> Vec<(&'static str, &'static str)>;
}

20+ Reusable Modules

Module	Description
borders	4 border styles (Rounded, Single, Double, ASCII) with RoundedBorder widget
focus	FocusManager for tab navigation with wrap-around
scroll	ScrollState and scrollbar rendering utilities
key_hints	KeyHintsBar for displaying keyboard shortcuts
text	StyledText builder and TextStyle presets
input	TextInput with grapheme-aware cursor positioning
selector	Full-featured selection list with search, shortcuts, disabled items
form	5 field types: Text, Secret, Number, Toggle, Select
modal	ModalStack for nested dialogs
dropdown	Dropdown state and rendering
checkbox	Checkbox component
radio	Radio button group
list	Simple list with selection
toast	Toast notifications with auto-expiry
spinner	Loading spinners with multiple styles
panel	Slide-in panels (left/right)
popup	Popup positioning utilities
card	Card container with title and hints

Theme Integration

Uses cortex-core's Ocean/Cyan theme constants:

• CYAN_PRIMARY, BORDER, BORDER_FOCUS
• SURFACE_0, SURFACE_1, SURFACE_2
• TEXT, TEXT_DIM, TEXT_MUTED

Testing

• 73 unit tests covering all major components
• All tests pass
• cargo check and cargo fmt clean

Future Work

Gradually migrate existing TUI widgets to use these components to reduce code duplication.

Checklist

• Code compiles (cargo check)
• Tests pass (cargo test -p cortex-tui-components)
• Formatted (cargo fmt)
• No breaking changes to existing code