Improve query editor execution and autocomplete input

[Maxteabag]

Summary

• forward configured query-insert actions before TextArea consumes editing keys
• preserve Enter/newline by default, but accept autocomplete without a newline when the user explicitly binds autocomplete_accept to Enter
• make autocomplete rows clickable and apply the clicked suggestion
• retain the existing Tab autocomplete behavior and Ctrl+Enter execution binding

Why

GNOME Terminal on Pop!_OS 22 uses the legacy terminal keyboard encoding where Ctrl+Enter and Enter are both sent as carriage return (^M). Sqlit cannot recover a modifier the terminal does not transmit. This PR does not add another execution shortcut; it instead ensures configured insert-mode actions are routed correctly on terminals that can report them.

Autocomplete handling is binding-aware: the default remains Tab and ordinary Enter remains a newline, while an explicit Enter autocomplete binding works as configured.

Partially addresses #299. Query tabs are deliberately excluded because they are separate product/lifecycle scope.

Validation

• focused keyboard/mouse tests: 23 passed
• full unit/UI suite before the final binding-specific adjustment: 1,416 passed, 2 skipped
• real Textual pilot.press() coverage for Ctrl+Enter, Enter, and Tab
• real Textual pilot.click() coverage for choosing a suggestion
• Ruff and structured autoreview: clean