fix: Ctrl+A/E navigate to line start/end instead of buffer

[remorses]

The default keybindings for Ctrl+A and Ctrl+E in TextareaRenderable were incorrectly mapped to buffer-home and buffer-end (jumping to the start/end of the entire textarea). This PR changes them to line-home and line-end to match standard Emacs/readline behavior used in virtually all Unix shells and TUI applications.

This is the standard behavior:

• Ctrl+A → beginning of current line
• Ctrl+E → end of current line

This fix also resolves unexpected behavior on macOS terminals. Many terminal emulators (including Ghostty, iTerm2, Terminal.app, and Zed's integrated terminal) translate Cmd+Left and Cmd+Right into Ctrl+A and Ctrl+E respectively. This is because macOS GUI applications use Cmd+Left/Right for line navigation, so terminals map these to the equivalent Unix keybindings. With the previous incorrect mapping, pressing Cmd+Left would unexpectedly jump to the start of the entire textarea instead of just the current line.

Tested on both Ghostty and Zed terminal on macOS.