Normalize PTY writes for PowerShell and route pastes through shared write path

[psklarkins]

Motivation

• PowerShell (pwsh/cmd) under ConPTY expects CR-driven input for Enter semantics, and sending raw LF/CRLF during paste/send causes cursor/echo corruption.
• Paste and buffer-chooser paths were writing directly to the PTY without the platform-aware normalization, leading to inconsistent behavior in PowerShell shells.

Description

• Added encode_text_for_pty(text: &str) and changed send_text_to_active to pre-encode outbound text and write the encoded bytes to PTYs so both single-pane writes and sync_input fanout use the same byte buffer.
• On Windows the encoder normalizes \n and \r\n into \r to emulate typed Enter behavior for pwsh/cmd under ConPTY, while non-Windows retains the original bytes.
• Updated the interactive buffer chooser paste path to call send_text_to_active instead of writing directly to the pane.
• Updated paste_latest to route through send_text_to_active so paste-buffer actions benefit from the same normalization.
• Files changed: src/input.rs, src/copy_mode.rs.

Testing

• Ran cargo fmt which completed successfully.
• Ran full test/compile checks in this Linux environment: cargo test / cargo check for the native build failed due to existing platform-specific issues unrelated to these changes (Windows-only mouse constants and an unrelated symbol in server.rs), so compilation could not be validated here.
• Attempted cargo check --target x86_64-pc-windows-msvc but the Windows target toolchain is not installed in this environment so the Windows-target check could not be executed.

Note: the changes are narrowly scoped to encode and route outbound text for PTY writes; the compilation failures observed during validation are due to pre-existing cross-platform build issues in this repository and not caused by the newline normalization changes.

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