feat: add color and icon support to toast notifications

[st0o0]

• Add optional Color and Icon properties to ToastOptions and ToastMessage, allowing callers to customize the
  border color and leading icon of toast notifications. Defaults remain unchanged (BrightGreen border, ✓ icon).
• Add a Toast Notifications gallery page to Termina.Demo.Gallery with presets for success, error, warning, info,
  and custom toasts — demonstrating all new options in an interactive list.
• Update docs/advanced/clipboard-and-feedback.md with a new Custom Colors and Icons section including usage
  examples.

Test plan

• Run Termina.Demo.Gallery, navigate to Toast Notifications, trigger each preset and verify
  color/icon/position render correctly
• Verify the default toast (no color/icon override) still renders with green border and checkmark
• Confirm existing clipboard page toasts are unaffected
• dotnet build passes with zero warnings