feat(renderer): pin orchestrator row in sidebar session tree

[AllBeingsFuture]

Closes #2804

Supersedes #2810 (same change, rebased onto current main after merge conflicts). The original PR branch lives on AgentWrapper/agent-orchestrator and this worker could not force-push there (Permission denied to AllBeingsFuture), so the rebased tip is on AllBeingsFuture:ao/agent-orchestrator-24/sidebar-orchestrator-row.

What

• Adds a pinned "Orchestrator" row at the top of every expanded project session group in the Electron sidebar.
• The row shows the same live status dot as worker sessions and navigates to the orchestrator session on click.
• If no live orchestrator exists, clicking the row spawns one and then navigates.

Shared helper / targeting fix

• Extracts useOpenOrchestrator so both the topbar "Orchestrator" button and the new sidebar row route through the same recency-based picker (newestActiveOrchestrator).
• This fixes the lexicographic-targeting issue described in Worker session orchestrator button navigates to lexicographically smallest orchestrator instead of most recent one #1362.
• Parent tracking from Orchestrator button on worker page links to wrong parent session #1211 (parentOrchestratorId) is not available in the Go rewrite domain yet, so this change does not attempt that backend fix.

Rebase conflict resolution (ShellTopbar.tsx)

Main had grown past the PR base (new shell-terminal control, board spawn error UI, "Board" root crumb, richer open-orchestrator telemetry). Resolution kept those main behaviors and still routed open/spawn through useOpenOrchestrator. The hook rethrows spawn failures so the board topbar can surface boardSpawnError.

Tests

• cd frontend && npm test -- --run src/renderer/components/Sidebar.test.tsx ✅ (38 tests)
• Component tests for the pinned row: presence/order, navigation, spawn fallback, status dot, and active selection.

Visual verification

ao preview is for previewing URLs in the desktop browser panel, not the app chrome, so it is not applicable for this sidebar UI change.