fix(sender): 超高解像度 JPEG のデコード失敗を createImageBitmap で救済

[kuu13580]

<img>.decode() は超高解像度画像（デコード後 RGBA がブラウザの 1 枚あたりデコード上限を超える、例: 9180x16320 ≈571MiB）で EncodingError を返す。上限の緩い createImageBitmap にフォールバックして救済し、?debug=true のログに画素数・デコード後バイトを出して原因を判別できるようにした。

Test plan

• 超高解像度 JPEG（〜150MP 級）を含む写真を送信画面で選択し、サムネイルが表示され送信できる

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⚙️ Run configuration

Configuration used: Organization UI

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Run ID: ccb5cd92-c33c-49c4-a4fc-a58eb30562e8

📥 Commits

Reviewing files that changed from the base of the PR and between c03198d and c4f3b40.

📒 Files selected for processing (1)

• frontend/src/lib/image-processing.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch bugfix/large-jpeg-decode-fallback

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