feat: フォーカス中ペインの下にプレビューを常時表示

[myuron]

概要

フォーカス中ペインの下に、カーソル行のファイル／ディレクトリ内容を常時表示するプレビュー領域を追加しました。

• Source フォーカス時は Source ペインの下、Dest フォーカス時は Dest ペインの下に表示
• 非フォーカス側は従来どおり全高で描画

表示内容

• テキストファイル: 内容（先頭からプレビュー領域に収まる行数）
• ディレクトリ: 中のエントリ一覧（ディレクトリは末尾に /）
• バイナリ／読み込み失敗／空: その旨のメッセージ

変更点

• fs.FileSystem に ReadFile を追加。OSFS 実装は先頭 64KiB までに制限して巨大ファイルでも安全
• internal/ui/preview.go に previewLines / isBinary / renderPreview を追加
• render() でフォーカス側を縦分割（ペイン約2/3・プレビュー約1/3、最小高さを確保）

テスト

• isBinary / previewLines（テキスト・バイナリ・ディレクトリ・読み込み失敗・空・nil）の単体テスト
• フォーカス側にプレビューが描画されることの統合テスト
• gofmt / golangci-lint / go test ./... / nix build すべてパス

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: e64ae6f2-07ac-4dc6-a3fc-ff25183bf19a

📥 Commits

Reviewing files that changed from the base of the PR and between 6c8ee4d and 133416a.

📒 Files selected for processing (11)

• .claude/plans/plan.md
• internal/fs/fs.go
• internal/fs/osfs.go
• internal/pane/pane_test.go
• internal/ui/keybinding.go
• internal/ui/layout.go
• internal/ui/model.go
• internal/ui/model_test.go
• internal/ui/preview.go
• internal/ui/preview_test.go
• internal/ui/render.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/preview-pane

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