Fix terminal confirmations, input responsiveness, and broker retention

[Marc-André Moreau (mamoreau-devolutions)]

Summary

• Enforce paste confirmation and implement configured window-close confirmation with asynchronous dialogs.
• Keep terminal input ordered and bounded without blocking the UI; make the Unix relay nonblocking and apply shutdown deadlines.
• Disable unsupported settings rather than presenting controls with no runtime effect.
• Bound broker response retention while preserving in-flight deduplication.
• Extract confirmation and lifecycle components from the main window and document behavior.

Validation

• Application build and strict native compilation succeeded.
• Windows: 305 tests passed, 9 platform skips.
• Linux/WSL: 11 tests passed.
• macOS was not tested.

Behavior notes

• Automatic close confirmation protects multiple running sessions.
• Cancelling an in-flight write terminates the session to avoid corrupted framing.