Move sidebar visibility and active panel state into the engine

[JDonaghy]

Problem

The engine doesn't own sidebar visibility or active panel. Each backend maintains its own:

• TUI: TuiSidebar { visible, active_panel: TuiPanel } — local struct with its own enum
• GTK: self.sidebar_visible: bool, self.active_panel: SidebarPanel — local fields

This forces ~200 lines of mapping in each backend:

• TuiPanel ↔ engine focus flags sync (sync_sidebar_focus())
• window_nav_overflow → sidebar focus (backend-level)
• dap_wants_sidebar → set visible (backend-level)
• ext_panel_focus_pending → set panel + visible (backend-level)
• Activity bar click → sidebar state mapping (~100 lines per backend)

Proposed fix

Add to Engine:

• sidebar_visible: bool
• active_sidebar: SidebarPanel
• Engine::set_sidebar_panel(panel) / Engine::toggle_sidebar()

Remove TuiPanel enum and TuiSidebar struct. Remove GTK-local sidebar fields. Backends become thin "if engine says show sidebar, show it" wiring.

This also makes window_nav_overflow, dap_wants_sidebar, and ext_panel_focus_pending engine-internal — no backend flag-checking needed.

Impact

~200 lines removed per backend. Prerequisite for zero-effort Windows backend.

Context

Identified in Session 370 backend dedup audit (Tier 4 — architectural).