gui: Wails v2 vertical MVP — bindings + UI

Go bindings (cmd/verstak-gui/app.go):
- Wire notes.Service, files.Service, actions.Service, worklog.Service
- CreateNote, ListNotes, ReadNote, SaveNote
- ListFiles, ListActions, RunAction
- ListWorklog, CreateWorklog
- Fix DTO mappings for core types

Frontend (frontend/src/):
- api/verstak.js: Wails v2 API wrapper (window.go.main.App)
- App.svelte: full rewrite with sidebar + 6 tabs + notes editor
- Section filtering → nodes by section
- Notes: create, open textarea editor, save, dirty tracking
- Quick actions, worklog entries, empty states
- Node creation modal with section select

Build:
  cd frontend && npm run build
  rm -rf cmd/verstak-gui/frontend-dist && cp -r frontend/dist cmd/verstak-gui/frontend-dist
  go build -tags 'gui production webkit2_41' -o verstak-gui ./cmd/verstak-gui
  ./verstak-gui

frontend/src/api/verstak.js

@@ -0,0 +1,43 @@
+// Wails v2 API wrapper — single frontend access point to Go backend
+
+function wailsCall(method, ...args) {
+  if (window.go && window.go.main && window.go.main.App) {
+    return window.go.main.App[method](...args)
+  }
+  return Promise.reject(new Error('Wails bindings not loaded'))
+}
+
+// Sections
+export const listSections = () => wailsCall('ListSections')
+
+// Nodes
+export const listNodesBySection = (section) => wailsCall('ListNodesBySection', section)
+export const listChildren = (parentID) => wailsCall('ListChildren', parentID)
+export const getNodeDetail = (id) => wailsCall('GetNodeDetail', id)
+export const createNode = (parentID, type, title, section) =>
+  wailsCall('CreateNode', parentID, type, title, section)
+export const deleteNode = (id) => wailsCall('DeleteNode', id)
+
+// Notes
+export const listNotes = (nodeID) => wailsCall('ListNotes', nodeID)
+export const createNote = (parentID, title) => wailsCall('CreateNote', parentID, title)
+export const readNote = (noteID) => wailsCall('ReadNote', noteID)
+export const saveNote = (noteID, content) => wailsCall('SaveNote', noteID, content)
+
+// Files
+export const listFiles = (nodeID) => wailsCall('ListFiles', nodeID)
+
+// Actions
+export const listActions = (nodeID) => wailsCall('ListActions', nodeID)
+export const runAction = (id) => wailsCall('RunAction', id)
+
+// Worklog
+export const listWorklog = (nodeID) => wailsCall('ListWorklog', nodeID)
+export const createWorklog = (nodeID, summary, minutes) =>
+  wailsCall('CreateWorklog', nodeID, summary, minutes)
+
+// Search
+export const search = (query) => wailsCall('Search', query)
+
+// System
+export const verstakVersion = () => wailsCall('VerstakVersion')