feat: sharp edge pipeline V02, tessellation presets, media cache-bust, GMSH plan

Sharp Edge Pipeline V02:
- export_step_to_gltf.py: replace BRep_Tool.Polygon3D_s (returns None in XCAF) with
  GCPnts_UniformAbscissa curve sampling at 0.3mm step — extracts 17,129 segment pairs
- Inject sharp_edge_pairs + sharp_threshold_deg into GLB extras (scenes[0].extras)
  via binary GLB JSON-chunk patching (no extra dependency)
- export_gltf.py: read schaeffler_sharp_edge_pairs from Blender scene custom props,
  apply via KD-tree to mark edges sharp=True + seam=True (OCC mm Z-up → Blender transform)
- tools/restore_sharp_marks.py: dual-pass (dihedral angle + OCC pairs), updated coordinate
  transform (X, -Z, Y) * 0.001

Tessellation:
- Admin UI: Draft / Standard / Fine preset buttons with active-state highlighting
- Default angular deflection: preview 0.5→0.1 rad, production 0.2→0.05 rad
- export_glb.py: read updated defaults from system_settings

Media / Cache:
- media/service.py: get_download_url appends ?v={file_size_bytes} cache-buster
- media/router.py: Cache-Control: no-cache for all download/thumbnail endpoints

Render pipeline:
- still_render.py / turntable_render.py: shared GPU activation + camera improvements
- render_order_line.py: global render position support
- render_thumbnail.py: updated defaults

Frontend:
- InlineCadViewer: file_size_bytes-aware URL update triggers re-fetch on regeneration
- ThreeDViewer: material panel, part selection, PBR mode improvements
- Admin.tsx: tessellation preset cards, GMSH setting dropdown
- MediaBrowser, ProductDetail, OrderDetail, Orders: various UI improvements
- New: MaterialPanel, GlobalRenderPositionsPanel, StepIndicator components
- New: renderPositions.ts API client

Plans / Docs:
- plan.md: GMSH Frontal-Delaunay tessellation plan (6 tasks)
- LEARNINGS.md: OCC Polygon3D_s None issue + GCPnts fix
- .gitignore: add backend/core (core dump from root process)

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

frontend/src/App.tsx

@@ -1,8 +1,9 @@
-import { BrowserRouter, Routes, Route, Navigate } from 'react-router-dom'
-import { useAuthStore } from './store/auth'
+import { BrowserRouter, Routes, Route, Navigate, useLocation } from 'react-router-dom'
+import { useAuthStore, isPrivileged as checkIsPrivileged } from './store/auth'
 import { WebSocketProvider } from './contexts/WebSocketContext'
 import Layout from './components/layout/Layout'
 import LoginPage from './pages/Login'
+import NotFoundPage from './pages/NotFound'
 import DashboardPage from './pages/Dashboard'
 import OrdersPage from './pages/Orders'
 import OrderDetailPage from './pages/OrderDetail'
@@ -27,14 +28,15 @@ import AssetLibraryPage from './pages/AssetLibrary'
 
 function ProtectedRoute({ children }: { children: React.ReactNode }) {
   const token = useAuthStore((s) => s.token)
-  if (!token) return <Navigate to="/login" replace />
+  const location = useLocation()
+  if (!token) return <Navigate to="/login" state={{ from: location.pathname }} replace />
   return <>{children}</>
 }
 
 function AdminRoute({ children }: { children: React.ReactNode }) {
   const { token, user } = useAuthStore()
   if (!token) return <Navigate to="/login" replace />
-  if (user?.role !== 'admin' && user?.role !== 'project_manager') return <Navigate to="/" replace />
+  if (!checkIsPrivileged(user)) return <Navigate to="/" replace />
   return <>{children}</>
 }
 
@@ -123,6 +125,7 @@ export default function App() {
             }
           />
         </Route>
+        <Route path="*" element={<NotFoundPage />} />
         </Routes>
       </WebSocketProvider>
     </BrowserRouter>