feat(workflow): webhook trigger with dual endpoint, path-based routing, and theme fixes

Backend:
- Add webhook_respond node: custom HTTP status/body/headers, _webhook_respond
  sentinel for reliable extraction, template rendering via RenderCtx
- Add Trigger.RespondMode (immediately/last_node/respond_node) with full doc
- Router: RunNowWithDone + DispatchWithDone for blocking webhook response
- Router: respondModeFor lookup, 404 on no-match path (draft + published)
- DraftWebhookHandler: path-match guard before run, method check
- writeWebhookResult: 502 when respond_node mode but no respond node ran
- Validator: warning when respond_node set but no webhook_respond reachable
  from trigger entry_node (BFS via existing BfsReachable)
- graph.entry dangling reference silently ignored (was warning, confused users)
- Publish preserves enabled flag from published copy
- spa_workflows: webhook-test source buckets to test KIND=test filter

Frontend:
- Trigger inspector: respond_mode radio picker (3 options with descriptions)
- NodeDetailModal: webhook_respond section (status, body, headers, ArgField)
- Canvas: trigger validation badges (error/warning) with click-to-inspector
- Canvas: edge hover highlight (emerald stroke), activeEdge persists during ctx menu
- Canvas: validate-on-load so badges appear immediately
- Canvas: "Open inspector" in context menu for nodes + triggers
- Canvas: layout fix — overflow-hidden on island wrapper prevents main scroll gap
- BottomTabs: remove border-b when collapsed, flush to canvas edge
- BaseNode: border-2 border-emerald-400 on selected (visible inside overflow-hidden)
- Editor stores: validate on loadWorkflow

Docs: remove all YAML examples from triggers.md (storage is JSON)