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>

backend/app/main.py

@@ -7,7 +7,7 @@ from fastapi.staticfiles import StaticFiles
 from sqlalchemy import text, inspect as sa_inspect
 
 from .database import Base, engine, SessionLocal
-from .routers import bundles, connectors, diagrams, devices, export, git_mgr, octopart, views, wires
+from .routers import bundles, connectors, diagrams, devices, export, formboard, git_mgr, octopart, views, wires
 
 Base.metadata.create_all(bind=engine)
 
@@ -70,6 +70,7 @@ app.include_router(views.router,     prefix="/api/views")
 app.include_router(octopart.router,  prefix="/api/octopart")
 app.include_router(export.router,    prefix="/api/export")
 app.include_router(connectors.router, prefix="/api/connectors")
+app.include_router(formboard.router,  prefix="/api/formboard")
 app.include_router(git_mgr.router,   prefix="/api/git")
 
 FRONTEND_DIR = os.path.normpath(