SidebarSystem migration: SC panel — TUI backend

[JDonaghy]

Summary

Wire the TUI backend to use the SC panel's SidebarSystem (added in the engine layer commit on #321).

What changes

Rendering (tui_main/panels.rs)

Replace the source_control_to_tree_view() + draw_tree() call in render_source_control() with:

• engine.sc_sidebar_body_rect.set(q_rect)
• render::populate_sc_sidebar_system(engine, theme)
• engine.sc_sidebar_system.borrow().render(b, q_rect)

Header, commit input, and button rows remain unchanged.

Key routing (tui_main/mod.rs)

Replace the ~80-line manual key mapping block (lines 2865–2948) with:

let result = engine.dispatch_sc_sidebar_key_unified(key_str, ctrl, unicode);
if matches!(result, ScKeyResult::Unfocused) {
    sidebar.has_focus = false;
}

Mouse handling (tui_main/mouse.rs)

• Scroll (~line 1234): Replace sc_selected ± 3 with scroll UiEvent through handle_cached.
• Click (~line 2478): Replace manual sc_visual_row_to_flat + sc_selected assignment with click UiEvent through handle_cached + dispatch_sc_sidebar_event(). Keep header/commit/button hit-testing.

Key files

• src/tui_main/panels.rs — SC render
• src/tui_main/mod.rs — SC key routing
• src/tui_main/mouse.rs — SC scroll + click

Prereqs

• Engine + render layer (this branch)

Testing

• cargo test --no-default-features — all tests pass
• Smoke test TUI: navigate j/k, Tab collapse, s/S stage, d discard, c commit, Enter open, q/Esc unfocus, mouse scroll/click, branch picker b/B