Image viewer: capture Escape only when the view changed; cover the hover and pan-only cases #52

Follow-up from the #51 review.

The viewer currently captures Escape when it is hovered OR its state is not the default. A hovered, untouched viewer still swallows Escape even though there is nothing to reset: for example, a modal that closes on Escape release stays open while the cursor is over an image inside it. Capturing only when the state is not the default would interfere less.

The tests in widget/tests/image_viewer.rs also leave two cases uncovered:

• the hover clause (viewer.rs:166-167);
• the offset part of is_default (viewer.rs:171), i.e. a pan-only state.

Mutation-check both new tests.