🎨 Apply smooth gradient interpolation across TUI elements

Refactor hardcoded gradient arrays into reusable utilities that generate
smooth color transitions per-character. This creates more visually
polished gradients that scale properly regardless of text length.

Add gradient presets (Aurora, Sunset, Emerald, Ember) and helper
functions (interpolate_color, generate_smooth_gradient, rgb_to_color)
to src/ui.rs for consistent SilkCircuit theming.

Apply the new gradient system to:
- Title bar and header elements
- Commit message display and labels
- Info bar user/email/preset fields
- Popup titles (emoji, preset, user info)
- Separator lines

Replace static color arrays with dynamic interpolation for smoother
visual transitions between brand colors.

Author: hyperb1iss