feat(open): implement interactive TUI file browser

[jmgilman]

Summary

• Add blob open command for interactively exploring blob archives
• Split-view TUI with file tree navigation and content preview
• Lazy loading via HTTP range requests for fast navigation
• File size guard prevents loading files >512KB into memory
• Status bar displays selected file metadata (size, modification time)

Features

• File tree navigation: Browse archive contents with keyboard (arrows, Enter, Tab)
• Content preview: Syntax highlighting for text, hex dump for binary files
• File metadata: Status bar shows file size and modification time (relative for recent, absolute for older)
• Copy to local: Press 'c' to copy selected file to local filesystem

Components

Package	Description
internal/tui/open	Main Bubble Tea model and update logic
internal/tui/components/filetree	File tree navigation component
internal/tui/components/preview	Content preview (text/binary)
internal/tui/components/statusbar	Status bar with file metadata
internal/tui/components/copydialog	File copy dialog
internal/tui/detect	Binary file detection

Test plan

• Run blob open docker.io/meigma/blob-test:v1 and verify TUI loads
• Navigate with arrow keys, verify file tree updates
• Select files and verify preview loads with syntax highlighting
• Select binary file and verify hex dump displays
• Verify status bar shows file size and modification time
• Press 'c' on a file and verify copy dialog appears
• Complete copy and verify file is written locally
• Select a large file (>512KB) and verify "Too Large" message appears

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