Fix terminal exit bugs, add help search, implement copy dialog options

[bluestreak01]

Summary

• Fix terminal exit requiring extra Enter — shell/claude panels now set dirty = true on process exit, so the UI redraws immediately instead of waiting for user input
• Fix terminal exit landing in editor — consequence of the above; the extra Enter was being processed by the file panel, accidentally opening files
• Add search to help dialog (F1) — type to filter entries with match highlighting, backspace to edit, all navigation still works
• Implement overwrite confirmation dialog — "Ask" mode now shows a red-themed per-file dialog with Overwrite/Skip/Skip All/Cancel options, works correctly for files inside directories
• Implement all copy dialog options:
  • Overwrite modes: Ask, Overwrite, Skip, Rename, Append
  • Sparse file copy (SEEK_HOLE/SEEK_DATA on Linux, zero-block detection on macOS)
  • Copy-on-write (clonefile() on macOS/APFS, FICLONE ioctl on Linux/btrfs)
  • Symlink modes: Smart (follow top-level, preserve inside dirs), Follow, Preserve
  • Permission and extended attribute preservation
  • Write cache bypass (F_NOCACHE on macOS)
  • Multiple destinations (semicolon-separated paths)
  • Produce sparse enabled by default
• Cursor positioning after mkdir (F7) — cursor jumps to the newly created directory

Test plan

• Open terminal (Ctrl+O), type exit, verify it closes immediately without extra Enter
• Verify focus returns to file panel after terminal exit
• Open help (F1), type a search term, verify filtering and highlighting
• Copy files with Ask overwrite mode, verify per-file dialog appears for conflicts
• Test Skip, Skip All, Cancel in overwrite dialog
• Copy with Rename mode to existing destination, verify auto-numbering
• Test symlink copy modes (create symlinks, copy with Smart/Follow/Preserve)
• Create directory (F7), verify cursor lands on new entry
• Copy to multiple destinations using semicolons

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