Add rectangular block selection and clipboard copy; refactor rendering and update deps

[enijburg]

Motivation

• Add support for rectangular (block) selections and enable copying block selections to the system clipboard.
• Simplify and centralize character rendering logic to correctly account for left-column clipping and visible width.
• Update dependency graph to pull in clipboard/graphics/image-related crates required for clipboard and block rendering features.

Description

• Introduced block selection state on Viewer with block_selection_anchor and block_selection_focus, plus a block_selection_range() accessor and line_display_chars() helper to map visual columns to characters.
• Implemented rectangular copy behavior in a new copy_selection_to_clipboard() function that handles both block and linear selections and updates viewer.status_message on success or failure.
• Updated mouse handling to start a block selection when left-clicking with Ctrl, to update block focus while dragging, and to copy on right-click; linear selection behavior is preserved otherwise.
• Refactored inline push_char logic into a top-level push_char() helper used by render_line() to handle left-column clipping, visible width limits, and segment building.
• Clear selection now resets both linear and block selection state, and render_line() will include block selection when computing highlight segments.
• Updated Cargo.lock with additional dependencies (clipboard/GUI/image/PNG/TIFF related crates and windows targets) required by the new clipboard and block selection implementation.

Testing

• Built the project with cargo build and the build completed successfully.
• Ran unit tests with cargo test and all tests passed.
• Performed integration sanity checks by running the application and exercising mouse-based linear and block selections and the copy-to-clipboard flow, with expected status messages appearing.

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Codex Task