fix(composer): forward Ctrl+Arrow to the terminal on Windows

[DocInTardis]

Summary

• forward Ctrl+Arrow from the Composer to the PTY on Windows, matching the existing macOS Cmd+Arrow behavior

Changes

• extract Composer passthrough key handling into getComposerPassthroughSequence()
• use the platform-aware primary modifier logic instead of checking event.metaKey directly
• keep plain arrow keys in the Composer when draft text is present
• add regression tests for Windows Ctrl+Arrow and plain-arrow behavior in tests/composer-input-behavior.test.ts

Testing

• node --experimental-strip-types --test tests/composer-input-behavior.test.ts

Closes #37