SidebarSystem migration: SC panel — GTK backend + cleanup

[JDonaghy]

Summary

Wire the GTK backend to use the SC panel's SidebarSystem (engine layer on #321, TUI on #339), then remove dead code.

What changes

Rendering (gtk/draw.rs)

Replace source_control_to_tree_view() + draw_tree() with populate_sc_sidebar_system() + sidebar.render().

Event routing (gtk/mod.rs)

• Key routing (~line 5566): Replace handle_sc_key() with dispatch_sc_sidebar_key_unified().
• Click (~line 9012): Keep header/commit/button hit zones. Replace the ~60-line section accumulator walk with handle_sc_sidebar_ui_event().
• Wire wire_da_events for the SC DrawingArea → Msg::ScSidebarEvent(UiEvent).

Cleanup (after both backends wired)

• Remove sc_selected, sc_sections_expanded, sc_has_focus from Engine.
• Remove sc_flat_len(), sc_flat_to_section_idx(), sc_visual_row_to_flat().
• Remove source_control_to_tree_view() from render.rs.
• Slim SourceControlData — remove sections_expanded, selected, has_focus.
• Update accessors.rs (sidebar_has_focus/clear_sidebar_focus), keys.rs (line 217 gate).
• Remove all #[allow(dead_code)] from the new SidebarSystem methods.
• Gate win-gui references behind #[cfg(feature = "win-gui")].
• Update SC tests to use SidebarSystem state.

Key files

• src/gtk/draw.rs, src/gtk/mod.rs — GTK rendering + events
• src/core/engine/mod.rs — remove old fields
• src/core/engine/source_control.rs — remove old helpers
• src/core/engine/accessors.rs, keys.rs, buffers.rs, ext_panel.rs — update refs
• src/render.rs — remove source_control_to_tree_view
• src/core/engine/tests.rs — update SC tests

Prereqs

• SidebarSystem migration: SC panel — TUI backend #339 (TUI backend)

Testing

• cargo test --no-default-features — all tests pass
• cargo clippy -- -D warnings — clean (no dead_code allows remain)
• Smoke test GTK: same interactions as TUI, plus mouse hover on buttons, double-click, scrollbar drag