Add STL thumbnail generation support

- Add trimesh and matplotlib dependencies for software-based 3D rendering
- Create stl_thumbnail service with generate_stl_thumbnail() function
- Handle mesh simplification for large files (>100k vertices)
- Auto-generate thumbnails during STL file upload and ZIP extraction
- Add POST /library/files/{id}/regenerate-thumbnail endpoint
- Add POST /library/generate-stl-thumbnails batch endpoint
- Add "Generate Thumbnails" button to file manager toolbar
- Add "Regenerate Thumbnail" option to file context menu
- Add unit and integration tests for new functionality

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -2821,6 +2821,21 @@ export const api = {
         used_meters: number;
       }>;
     }>(`/library/files/${fileId}/filament-requirements${plateId !== undefined ? `?plate_id=${plateId}` : ''}`),
+
+  // STL Thumbnail Generation
+  regenerateFileThumbnail: (fileId: number) =>
+    request<LibraryFile>(`/library/files/${fileId}/regenerate-thumbnail`, {
+      method: 'POST',
+    }),
+  batchGenerateStlThumbnails: (options: {
+    file_ids?: number[];
+    folder_id?: number;
+    all_missing?: boolean;
+  }) =>
+    request<BatchThumbnailResponse>('/library/generate-stl-thumbnails', {
+      method: 'POST',
+      body: JSON.stringify(options),
+    }),
 };
 
 // AMS History types
@@ -3045,6 +3060,21 @@ export interface ZipExtractResponse {
   errors: ZipExtractError[];
 }
 
+// Batch Thumbnail Generation types
+export interface BatchThumbnailResult {
+  file_id: number;
+  filename: string;
+  success: boolean;
+  error: string | null;
+}
+
+export interface BatchThumbnailResponse {
+  processed: number;
+  succeeded: number;
+  failed: number;
+  results: BatchThumbnailResult[];
+}
+
 // Library Queue types
 export interface AddToQueueResult {
   file_id: number;