RichTextPopup: scrollbar drag + track-click not wired in either backend

[JDonaghy]

Problem

After #214's first cut, the popup's scrollbar is rendered with correct geometry (track + thumb proportional to content) on both TUI and GTK, but neither backend wires up drag or track-click handlers for it. Mouse wheel scrolling works (existing engine path); only the bar itself is non-interactive.

Repro

1. Trigger an LSP hover popup on a long doc string (any std lib symbol works).
2. Try to drag the scrollbar thumb — nothing happens.
3. Try to click the track outside the thumb to page-jump — nothing happens.

Fix shape

Mirror the pattern from #cb84f82 / #3e5d7d3 for sidebar scrollbars:

• The popup primitive's layout already returns Option<PopupScrollbar> with track + thumb Rects.
• Backends record the scrollbar bounds in a per-frame cell.
• Click handler: if click lands on track, jump-scroll. If on thumb, arm DragState::ScrollbarY with the popup's id.
• dispatch_mouse_drag already does the math correctly.
• On UiEvent::ScrollOffsetChanged { widget, new_offset } where widget == popup id, update engine.editor_hover.scroll_top.

Files

• quadraui/src/primitives/rich_text_popup.rs — already exposes PopupScrollbar + RichTextPopupHit::ScrollbarThumb / ScrollbarTrack.
• src/tui_main/mouse.rs — wire popup scrollbar drag + track click.
• src/gtk/mod.rs — wire popup scrollbar drag + track click. Cell width: pass through from draw_rich_text_popup's render so click handler knows the actual scrollbar pixel bounds.
• src/core/engine/panels.rs — editor_hover.scroll_top setter.

Surfaced by

#214 first-cut smoke test.

[JDonaghy]

Implemented in db723e0 on develop. Track click + thumb drag now route through quadraui::DragState::ScrollbarY + dispatch_mouse_drag. GTK paint and hit-test geometry share constants so they no longer drift.