feat: integrate graphic element detection into structure step

Add cv_graphic_detect.py for detecting non-text visual elements (arrows,
circles, lines, exclamation marks, icons, illustrations). Draw detected
graphics on structure overlay image and display them in the frontend
StepStructureDetection component with shape counts and individual listings.

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

admin-lehrer/app/(admin)/ai/ocr-pipeline/types.ts

@@ -213,12 +213,25 @@ export interface RowGroundTruth {
   notes?: string
 }
 
+export interface StructureGraphic {
+  x: number
+  y: number
+  w: number
+  h: number
+  area: number
+  shape: string   // arrow, circle, line, exclamation, dot, icon, illustration
+  color_name: string
+  color_hex: string
+  confidence: number
+}
+
 export interface StructureResult {
   image_width: number
   image_height: number
   content_bounds: { x: number; y: number; w: number; h: number }
   boxes: StructureBox[]
   zones: StructureZone[]
+  graphics: StructureGraphic[]
   color_pixel_counts: Record<string, number>
   has_words: boolean
   word_count: number