feat(app): send Enter after send-selection paste

[simota]

Summary

Adds the noa-specific config key send-selection-send-enter (default false). When enabled, the Send Selection to Pane picker follows the paste into the target pane with an Enter, so the sent selection is submitted immediately.

• Atomic delivery: the Enter is appended to the encoded paste outside the bracketed-paste wrapper and written as one pty queue element, so a backpressured queue admits or drops the pair atomically — a lone Enter after a dropped paste would execute the stale prompt line. Skipped when nothing was pasted, for the same reason.
• Protocol-faithful encoding: the trailing Enter goes through the real key encoder under the target pane's Kitty keyboard flags (CSI 13 u under report-all-keys, legacy CR otherwise), so Kitty-protocol TUIs recognize it.
• Paste protection: when clipboard-paste-protection raises the confirm dialog, the Enter is deferred with the paste and sent only on confirm.
• Settings UI: also editable from the Settings overlay as a new "Send Selection Enter" row (bool toggle, on-save, mirrored into the running config on commit and undo — same shape as Confirm Quit). Normal cmd+v paste behavior is unchanged.

Test plan

• cargo test -p noa-app (811) / cargo test -p noa-config (167) all green; cargo clippy --workspace clean (full-workspace ipc/pty suites verified unsandboxed)
• New tests: config default/precedence/parse, encode_enter_key vs Kitty flags, settings-row toggle→commit key/value proof, revert/undo config mirroring