fix(desktop): allow restarting saved relay-mesh agents from the UI

[tlongwell-block]

Problem

Restarting a saved relay-mesh agent (Spawn/Respawn in the UI) fails with:

Relay-mesh agents need a fresh serve target before start. Create a new agent with Run on relay mesh selected.

forcing users to delete and recreate the agent on every restart.

Root cause

The error comes from a frontend-only guard, relayMeshAgentError() in managedAgentControlActions.ts, which predates #879. It was correct back when the serve target's dial pointer was only captured at creation time and never persisted — a saved start genuinely couldn't work.

#879 moved that responsibility into the Rust start preflight: ensure_relay_mesh_for_record re-resolves a live bootstrap target from the relay's gossiped serve targets, brings up the local client node, and publishes the paired kind:24621 connect-request so the peer dials back. Every UI start path (start_managed_agent → start_local_agent_with_preflight with allow_fresh_create_start=false) already runs this — the app-relaunch restore path uses it today and restarts mesh agents fine. The stale TS guard just rejected the UI start before the backend ever saw it.

Fix

• Remove the guard from startManagedAgentWithRules / respawnManagedAgentWithRules; the backend preflight decides, and its actionable errors ('no live serve target for this model' vs 'could not refresh targets') surface through the existing error-toast path. No Rust changes needed.
• Update the e2e mock bridge (handleStartManagedAgent) to model the real preflight: start succeeds when a live serve target exists for the agent's model, fails with the backend's actual error copy when none does.
• Rewrite the e2e test to cover both outcomes (restart succeeds with a live target; actionable failure + agent stays stopped without one) and the unit test to verify delegation + error propagation.

Verification

• pnpm test (desktop): 624/624 pass
• pnpm typecheck, pnpm check: clean (1 pre-existing warning on main, untouched)
• playwright test tests/e2e/mesh-compute.spec.ts: 7/7 pass