feat(tui-components): add ASCII preview generator and UI documentation (#469)

- Add dump_previews example to generate ASCII art previews of all UI components
- Create ui-previews/ directory with markdown documentation for each component
- Generate COMPONENTS.md with a complete gallery of all component previews
- Components documented: Card, Modal, Dropdown, TextInput, Panel, Selector,
  CheckboxGroup, RadioGroup, ScrollableList, Toast, LoadingSpinner, KeyHintsBar,
  Popup, Form, and Border Styles
- Fix unused mut warning in modal.rs test

Run with: cargo run -p cortex-tui-components --example dump_previews

Co-authored-by: Droid Agent <droid@factory.ai>

Author: echobt

cortex-tui-components/Cargo.toml

@@ -25,5 +25,9 @@ serde = { workspace = true }
 [dev-dependencies]
 tokio-test = { workspace = true }
 
+[[example]]
+name = "dump_previews"
+path = "examples/dump_previews.rs"
+
 [lints]
 workspace = true