Add mobile camera capture, polaroid preview, gallery save & native share

- カメラ撮影 → プレビュー(ポラロイド風白縁)→ 保存 / ネイティブ共有
- iOS / Android のネイティブ実装(共有・ギャラリー保存・FileProvider)
- iOS は共有シートのプリウォームで初回表示を軽減
- iOS Info.plist 用途説明をビルド後処理で自動付与
- Android パッケージ名ガードを有効化(ビルド前チェック)
- README を整備、.gitignore に slnx / .vscode / .utmp を追加

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ProjectSettings/NavMeshAreas.asset

@@ -0,0 +1,91 @@
+%YAML 1.1
+%TAG !u! tag:unity3d.com,2011:
+--- !u!126 &1
+NavMeshProjectSettings:
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+  m_LastAgentTypeID: -887442657
+  m_Settings:
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+    accuratePlacement: 0
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+  m_SettingNames:
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