[Feature Request] Configurable keybindings — e.g. remap "insert newline" to Alt+Enter

Problem

In the TUI input box, inserting a newline is only possible via Shift-Enter or Ctrl-J. Many users have muscle memory for Alt+Enter from other tools (chat apps, editors, other CLIs). Currently there is no way to remap keys: neither config.toml nor tui.toml exposes keybinding settings, and the keyboard-shortcuts docs list fixed shortcuts only.

Proposed solution

Support user-configurable keybindings for TUI input actions — at minimum for "insert newline" — e.g. a [keybindings] table in tui.toml or a dedicated keybindings.toml. Alt+Enter should be bindable as an alternative newline key.

Alternatives considered

Terminal-level remapping (kitty / WezTerm can translate Alt+Enter into the Shift-Enter escape sequence), but it is terminal-specific and not portable.

Additional context

Related: #256 — newline key handling is already a pain point on some platforms; configurable bindings would give users a fallback when the default doesn't work in their terminal.

Environment: kimi 0.26.0, Linux

Start by locating the TUI input box key handling and the existing config.toml or tui.toml parsing, then review the keyboard-shortcuts documentation. Done means users can configure TUI input actions, including binding Alt+Enter as an alternative for inserting a newline, with the supported configuration documented.