Add mouse/keyboard text selection with clipboard copy feedback

[enijburg]

Motivation

• Add interactive text selection so users can select and copy bytes from the memory-mapped view with both mouse and keyboard.
• Provide visible feedback (reversed video) for selection and status messages when copying succeeds or fails.

Description

• Introduce selection state and status text to Viewer (cursor_offset, selection_anchor, selection_focus, status_message) and wire selection-aware rendering so selected ranges render with reverse-video alongside existing search highlights.
• Implement mouse-based selection: click to start, drag to extend, and release to finish selection with a footer status message showing selection size and instructions.
• Implement keyboard selection extension with Shift+Left/Right to adjust selection and Ctrl+C to copy the selected bytes to the system clipboard using the arboard crate, with success/failure messages shown in the footer.
• Add helper functions selection_range, clear_selection, and offset_for_line_col to map screen coordinates and tab-aware columns to byte offsets for accurate selection.

Testing

• Ran cargo fmt successfully in the workspace.
• Attempted cargo test, but it failed to fetch crates from crates.io due to network/proxy restrictions in the environment (failed downloading config.json), so unit tests were not executed here.

---

Codex Task