Replace arboard clipboard backend with copypasta

[enijburg]

Motivation

• Replace the arboard clipboard dependency with the cross-platform copypasta crate to simplify platform-specific transitive dependencies and unify clipboard API usage.

Description

• Add copypasta = "0.10" to Cargo.toml and remove the direct arboard dependency from the project manifest.
• Update src/main.rs to use copypasta::{ClipboardContext, ClipboardProvider} instead of arboard::Clipboard and change initialization from Clipboard::new() to ClipboardContext::new().
• Adjust the clipboard helper copy_selection_to_clipboard to accept Option<ClipboardContext> and replace calls to set_text with set_contents to match the copypasta API.
• Regenerate Cargo.lock, removing many platform-specific crates that were pulled in by the previous clipboard backend and updating windows-sys entries.

Testing

• Ran cargo build which completed successfully.
• Ran cargo test which completed successfully (no failing unit tests in the changed code paths).

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Codex Task