Rename IslandType::Floating to IslandType::Secondary

Problem/Motivation

Since #3630345: Preview as a switch on the Canvas, and in a new tab, instead of a split view, the islands of type IslandType::Floating sit in the header row of their panel: the viewport switcher, Highlight, Preview and Scaffold options. None floats anymore, but the enum, the profile form and the CSS still say so.

Proposed resolution

Rename the case IslandType::Secondary, value secondary, listed as Secondary controls in the profile form. The box pinned over a panel, left to contrib islands, follows with its class, slot and z-index token.

Remaining tasks

• Review.
• Publish the change record on merge.

User interface changes

The profile form lists Secondary controls instead of Floating controls.

API changes

IslandType::Floating becomes IslandType::Secondary. The wrapper class becomes db-island-secondary and the test IDs start with secondary_. The pinned box becomes db-island-secondary-controls, its slot view_secondary_controls, its token --db-z-secondary-controls. Renamed outright, as every API change before beta9, with a change record.

Data model changes

None: profiles store no island type.