fix: Move overlay from top to bottom of screen

HuggingFace Spaces header was causing top overlay to be cut off.
Moved OCR results overlay to bottom of video frame.

Rules:
- Always use git diff before commits
- Fix one issue at a time

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app.py

@@ -135,13 +135,16 @@ def detection(image):
         num_lines = 1 + min(len(ocr_texts), 5)  # Frame + OCRテキスト（最大5行）
         box_height = 30 + num_lines * 25
 
+        # 画面下部に配置（HF Spacesのヘッダーで上部が見切れるため）
+        y_start = h - box_height - 20
+
         # 背景（黒で塗りつぶし）
-        cv2.rectangle(output, (20, 20), (w - 20, 20 + box_height), (0, 0, 0), -1)
+        cv2.rectangle(output, (20, y_start), (w - 20, h - 20), (0, 0, 0), -1)
         # 緑の枠線
-        cv2.rectangle(output, (20, 20), (w - 20, 20 + box_height), (0, 255, 0), 2)
+        cv2.rectangle(output, (20, y_start), (w - 20, h - 20), (0, 255, 0), 2)
 
         # Frameカウンター
-        y_pos = 50
+        y_pos = y_start + 25
         cv2.putText(output, f"Frame:{_frame_count}", (35, y_pos),
                     cv2.FONT_HERSHEY_SIMPLEX, 0.7, (0, 255, 0), 2, cv2.LINE_AA)