feat: NetUP via agent — proxy HTTP requests through Windows workstation

- Migration 044: add connection_type + agent_workstation_id to netup_settings
- Backend: netupReqViaCfg routes requests via agent WebSocket when connection_type=agent
- Agent: add netup_request command handler (proxies HTTP to local NetUP server)
- TvWelcomePage: add connection mode toggle (direct / via agent) + workstation selector
- Hint changes: URL placeholder shows local IP when agent mode selected

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/044_netup_agent.sql

@@ -0,0 +1,4 @@
+-- NetUP connection via agent
+ALTER TABLE netup_settings
+  ADD COLUMN IF NOT EXISTS connection_type VARCHAR(10) NOT NULL DEFAULT 'direct',
+  ADD COLUMN IF NOT EXISTS agent_workstation_id UUID REFERENCES workstations(id) ON DELETE SET NULL;