TUI: Add ext_panel_to_tree_view() builder and migrate Extension Panel to TreeView

[JDonaghy]

Tier: Ready now (no quadraui gap — render.rs builder + TreeView primitive both exist)

Summary

render_ext_panel() in src/tui_main/panels.rs:1173–1525 (~326 lines, 70+ `set_cell` calls per row) hand-rolls flat-list construction, row rendering, section expansion, color dispatch by hint marker, icon padding, badge rendering, action labels, and a manual scrollbar. The shared ext_sidebar_to_tree_view() builder in render.rs:7331 is the right shape but operates on ExtSidebarData, not the distinct ExtPanelData type (render.rs:1639).

What exists

• render::ext_sidebar_to_tree_view(ext: &ExtSidebarData) -> quadraui::TreeView — already adapts Extensions sidebar to TreeView (quadraui: MultiSectionView primitive — eliminate per-backend section-walk drift #293 landed this).
• quadraui::TreeView + TreeController::render() — the primitive both backends draw through.
• populate_ext_sidebar_system() — engine-state-to-controller wiring pattern used by the Extensions sidebar.

What to do

1. Add render::ext_panel_to_tree_view(panel: &ExtPanelData, theme: &Theme) -> quadraui::TreeView (or MultiSectionView if sections-with-headers are needed) modeled on the existing ExtSidebar builder.
2. Map ExtPanel-specific concepts (sections, chevrons, badges, action labels, ExtPanelStyle) into TreeView nodes + the controller's badge/action node fields. If anything doesn't fit, file a quadraui gap.
3. Replace render_ext_panel() body with: build via the new fn, render via Backend::draw_tree_view() (or equivalent), route hit-tests through the layout's hit_test().
4. Move scroll/expansion state ownership to the engine (the sidebar version already uses ext_panel_tree_expanded — generalize if needed).

Files

• src/render.rs — add new builder.
• src/tui_main/panels.rs — delete most of render_ext_panel() body (~326 lines down to ~30–50 lines of wiring).
• Likely also src/gtk/draw.rs — switch GTK to the same builder for symmetry (the GTK path is out of scope for the issue body but the builder will be shared).

Dependencies

None — both render.rs builder and quadraui::TreeView are existing infrastructure. Sibling to GTK's same migration.

Notes

Part of the TUI convergence backlog (#474). The biggest single dedup in panels.rs. Confirm whether ExtPanel data needs TreeView or MultiSectionView — the sidebar uses both flavors (#293 introduced the multi-section variant).

[JDonaghy]

Landed in PR #482. ~252 lines of TUI-specific code removed.