Add formboard harness layout designer

Separate read-only canvas for physically laying out wire harnesses.
Syncs connectors from the main diagram (one-way), no changes flow back.

Backend:
- FormboardLayout, FormboardNode, FormboardSegment models
- /api/formboard/ router: get/create layout, sync connectors from diagram,
  full CRUD for nodes and segments

Frontend (formboard.js):
- Konva canvas: drag nodes, click-to-connect segments, zoom/pan
- Node types: Connector (blue rect, synced from diagram), Branch (green
  circle, manual), Splice/joint (orange diamond, manual)
- Segments scale thickness by wire count; fitting overlays for open,
  split loom, conduit, heat shrink, tape wrap
- Right-click context menu for delete on nodes/segments

App integration:
- "⊞ Formboard" tab in view-tab-bar (distinct color, independent of views)
- Sub-toolbar: Select / Branch / Splice / Connect / Sync / Fit / Delete
- Right panel swaps to formboard props when active:
  - Node: label, notes, linked device info
  - Segment: fitting type, fitting color, label, length, wire checklist
    (checkboxes for every wire in the main diagram)

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

frontend/js/api.js

@@ -60,6 +60,16 @@ const api = {
     jsonUrl: (id) => `${API_BASE}/export/${id}/json`,
     formboardUrl: (id) => `${API_BASE}/export/${id}/formboard`,
   },
+  formboard: {
+    get:           (diagramId)       => apiFetch(`/formboard/${diagramId}`),
+    sync:          (diagramId)       => apiFetch(`/formboard/${diagramId}/sync`, { method: "POST" }),
+    createNode:    (diagramId, data) => apiFetch(`/formboard/${diagramId}/nodes`,    { method: "POST", headers: { "Content-Type": "application/json" }, body: JSON.stringify(data) }),
+    updateNode:    (nodeId, data)    => apiFetch(`/formboard/nodes/${nodeId}`,        { method: "PATCH", headers: { "Content-Type": "application/json" }, body: JSON.stringify(data) }),
+    deleteNode:    (nodeId)          => apiFetch(`/formboard/nodes/${nodeId}`,        { method: "DELETE" }),
+    createSegment: (diagramId, data) => apiFetch(`/formboard/${diagramId}/segments`, { method: "POST", headers: { "Content-Type": "application/json" }, body: JSON.stringify(data) }),
+    updateSegment: (segId, data)     => apiFetch(`/formboard/segments/${segId}`,     { method: "PATCH", headers: { "Content-Type": "application/json" }, body: JSON.stringify(data) }),
+    deleteSegment: (segId)           => apiFetch(`/formboard/segments/${segId}`,     { method: "DELETE" }),
+  },
   git: {
     status:  ()                             => apiFetch("/git/status"),
     log:     (diagram_id)                   => apiFetch(diagram_id != null ? `/git/log?diagram_id=${diagram_id}` : "/git/log"),