Make Ctrl+F/Ctrl+B input-cursor movement configurable

Problem

Ctrl+F / Ctrl+B behavior is hardcoded and cannot be configured.

Today they are hardcoded to word motion, but single-character movement is the default behavior in Emacs, in every readline-based shell, and in the standard text fields of macOS apps (Cocoa's built-in Emacs-style keybindings). Users who expect Ctrl+F / Ctrl+B to move by one character cannot get that behavior through any [keybindings] setting; changing it currently requires editing Rust code.

Current behavior

In crates/jcode-tui/src/tui/app/input.rs, handle_control_key:

KeyCode::Char('b') => {
    if app.cursor_pos > 0 {
        app.cursor_pos = app.find_word_boundary_back();
    }
    true
}
KeyCode::Char('f') => {
    if app.cursor_pos < app.input.len() {
        app.cursor_pos = app.find_word_boundary_forward();
    }
    true
}

So jcode's Ctrl+F / Ctrl+B behave like Emacs M-f / M-b (word motion) rather than C-f / C-b (single character). Single-character movement currently exists only as bare Left/Right arrows in handle_basic_key. Word motion already exists as Alt+F / Alt+B in handle_alt_key.

The [keybindings] config (KeybindingsConfig in crates/jcode-config-types/src/lib.rs) covers scroll, model/effort switching, workspace nav, and pane toggles, but has no fields for input-editor motion.

Desired behavior

Make input-editor cursor movement configurable so Ctrl+F / Ctrl+B can be set to single-character movement, for example:

[keybindings]
cursor_left  = "ctrl+b"   # backward-char
cursor_right = "ctrl+f"   # forward-char
word_left    = "alt+b"    # backward-word (current behavior)
word_right   = "alt+f"    # forward-word (current behavior)
cursor_home  = "ctrl+a"
cursor_end   = "ctrl+e"

This keeps the motivating use case (Emacs/readline/macOS-style Ctrl+F/Ctrl+B as single-character movement) while leaving the shipped default as a separate decision.

Proposal

1. Add input-motion fields to KeybindingsConfig (and the default registry in crates/jcode-config-types/src/keybindings.rs).
2. Route handle_control_key / handle_alt_key's b/f (and a/e) behavior through those configured chords instead of hardcoding them.
3. Honor the existing config hot-reload path (refresh_keybindings_if_config_reloaded).
4. Document the new keys in the default config template and keybinding docs.

Notes / decisions needed

• Whether to change the shipped default for Ctrl+F / Ctrl+B to single-character movement, or keep the current word-motion default and only make it configurable.
• Whether to also expose kill/delete and history navigation as configurable keys, or keep this issue scoped to cursor movement only.

Start with handle_control_key and handle_alt_key in crates/jcode-tui/src/tui/app/input.rs, then inspect KeybindingsConfig and the default registry in crates/jcode-config-types/src/lib.rs and keybindings.rs. Review the default config template, keybinding documentation, and refresh_keybindings_if_config_reloaded. Done means cursor-motion chords are configurable, hot reload is honored, and the chosen default behavior is documented.