Full-height navigation zones in lightbox

[Darkkal]

Parent Issue

Part of #74 — More controls for media in lightbox

Problem

Nav buttons currently sit inside .mainArea and reposition when media aspect ratios change between posts, causing the user to "chase" the button.

Proposed Design

• Replace the current ‹ / › buttons with full-height click zones spanning the left/right edges of .mainArea, separate from and layered over the media
• Zones should be visually subtle: transparent by default, with a slight gradient reveal + chevron icon on hover
• Zone width: ~60px desktop, ~44px mobile (minimum touch target)
• Zones sit in front of the media but behind the controls bar and sidebar

Acceptance Criteria

• Navigation zones span the full height of the lightbox media area
• Zones stay in a fixed position regardless of media dimensions
• Hover state reveals a subtle directional gradient + chevron
• Prev zone hidden when at the first item; next zone hidden when at the last item with no more pages
• Next zone shows loading spinner when isPageLoading
• Mobile swipe gestures remain unchanged
• Zones do not overlap the sidebar panel or the top-right action controls

Edge Cases

• On desktop, zones stop at the sidebar edge — they don't extend behind it
• On mobile with the bottom sheet open, zones don't extend into the bottom sheet area
• Touch targets are at least 44×44px per accessibility guidelines

[Darkkal]

implemented and verified in #132