Make the floating table-of-contents toggle visible in preview mode

[PathGao]

Problem

The collapsed table-of-contents control is a bare chevron in preview-only mode. It is easily confused with nearby toolbar affordances and has too little visual separation from the document canvas.

Expected behavior

Keep the existing compact control and behavior, but render it as a subtle translucent rounded square in both preview-only and edit modes. Preview-only should remain quieter than hover/active states.

Scope

• Preserve the 28px target, side placement, open/close animation, icon direction, tooltip, translated accessible label, and print exclusion.
• Use existing visual tokens only.
• Do not add a label, setting, component, or behavior change.

Validation

Verify in an isolated macOS bundle in preview-only and edit modes, in addition to the source-level regression test.