Add reveal toggle for Anthropic API key setting

[bobthearsonist]

Problem

The Visual Notes settings page stores the Anthropic API key in a password field. When users paste the wrong value, there is no safe way to briefly reveal and verify the prefix/shape without replacing the key or inspecting plugin data manually.

Proposal

Add a reveal/hide control next to the Anthropic API key field.

Acceptance criteria

• The key remains hidden by default.
• A user can toggle the field between password and text display.
• The toggle is clearly labeled and accessible by keyboard/screen readers.
• The plugin should still avoid logging or displaying the key anywhere else.
• Existing save behavior remains unchanged.