Review UI refinements: separator styling and header layout

[backnotprop]

Summary

• Slimmer hunk separators: Reduce height from 32px to 24px, use semi-transparent theme-integrated backgrounds via Pierre's --diffs-bg-separator-override, fade text/buttons with opacity for a subtler look
• Header layout: Move sidebar toggles (Annotations, AI, Agents) to the far right, options menu to their left. Add divider between file tree button and repo label. Remove worktree pill from header
• Collapse viewed files: Viewed files in all-files review mode are now collapsible
• Fix gutter drag multi-line selection: Switch from deprecated enableHoverUtility + renderHoverUtility to enableGutterUtility + onGutterUtilityClick — the old API never entered Pierre's gutterSelecting mode, so dragging the gutter button always reported a single line

Closes #679
Closes #682

Test plan

• Open code review in split and unified mode — separators should span full width in both
• Verify separator buttons (expand up/down/both) still work with full file content
• Check header button order: actions → menu → sidebar toggles
• Light and dark themes both look correct for separator color
• Gutter button click-and-drag selects multiple lines (annotation shows full range)
• Single gutter click still works for single-line annotations