Make the floating TOC toggle visible in preview mode

[PathGao]

Summary

• give the floating table-of-contents toggle a subtle translucent rounded surface in preview-only and edit modes
• keep its 28px geometry, side placement, animation, icon direction, tooltip, accessible label, and behavior unchanged
• add a regression test that prevents the control from returning to an edit-only card or a transparent bare icon

Why

In preview-only mode the collapsed TOC control was only a bare chevron. It blended into the document canvas and was easy to confuse with nearby toolbar affordances. The new base surface is intentionally quieter than hover/active states while remaining visible through its translucent canvas, border, and shadow.

Validation

• npm run check (0 errors, 0 warnings)
• npm test (151 passed)
• cargo test (27 passed)
• isolated macOS release bundle: verified the collapsed control in preview-only and edit modes, then opened/closed the TOC without changing the installed app

Fixes #330.

Chain

Depends on #329. Please merge after #329; this is a focused visual follow-up and remains draft until its parent lands.

[PathGao]

@alecdotdev This draft is the next linear follow-up after #329 and fixes #330. It makes the collapsed TOC control subtly visible in preview-only mode without changing its 28px target, placement, animation, icon direction, or behavior. The isolated macOS bundle verified preview-only and edit-mode opening/closing. Please merge after #329.