feat: support cross-platform file and image paste in fullscreen input

[shuyhere]

Summary

BB-Agent fullscreen input currently does not support reliable cross-platform file and image paste/attach behavior.

Users cannot consistently paste or drop files/images into the input block across macOS, Linux, and Windows.

Current problems

• Ctrl+V is currently intercepted for image clipboard handling instead of falling back to normal paste behavior.
• Clipboard image paste is effectively Linux-only today (wl-paste, xclip, grab-screenshot).
• Pasted image-path detection is too strict and misses common terminal/drop formats:
  • quoted paths
  • escaped paths
  • file:// URLs
  • multiple paths
• There is no real general file-paste pipeline for non-image files.
• Fullscreen prompt submission does not mirror CLI @file processing, so pasted file references are not handled consistently.

Expected behavior

• Normal text paste should keep working everywhere.
• If an image is available from the clipboard, BB should support attaching it cross-platform.
• If a pasted/dropped value is a local file path, BB should normalize and recognize it.
• File/image paste behavior should work consistently in the fullscreen input block across macOS, Linux, and Windows.
• Fullscreen should support a real file-ingest flow comparable to CLI @file handling, or an equivalent attachment/reference flow.

Proposed implementation

• Make Ctrl+V fall back to standard paste if no image clipboard payload is available.
• Add macOS clipboard image support.
• Add Windows clipboard image support.
• Normalize pasted file/image paths:
  • quotes
  • escaped spaces
  • file:// URIs
  • multiple dropped paths
• Add fullscreen file ingestion/reference handling for pasted files.
• Add tests for path normalization and fullscreen paste submission behavior.

Acceptance criteria

• Text paste still works normally.
• Image clipboard paste works on macOS, Linux, and Windows.
• Drag/paste of image file paths attaches supported images.
• Drag/paste of normal files inserts/expands a usable file reference flow.
• Fullscreen input behavior matches user expectations across major terminals/platforms.