feat(tui): add Emacs keymap profile with runtime switching

- Introduce built‑in Emacs keymap (`keymap_emacs.toml`) alongside existing Vim layout.
- Add `ui.keymap_profile` and `ui.keymap_path` configuration options; persist profile changes via `:keymap` command.
- Expose `KeymapProfile` enum (Vim, Emacs, Custom) and integrate it throughout state, UI rendering, and help overlay.
- Extend command registry with `keymap.set_vim` and `keymap.set_emacs` to allow profile switching.
- Update help overlay, command specs, and README to reflect new keybindings and profile commands.
- Adjust `Keymap::load` to honor preferred profile, custom paths, and fallback logic.

crates/owlen-core/src/config.rs

@@ -1584,6 +1584,8 @@ pub struct UiSettings {
     pub show_timestamps: bool,
     #[serde(default = "UiSettings::default_icon_mode")]
     pub icon_mode: IconMode,
+    #[serde(default = "UiSettings::default_keymap_profile")]
+    pub keymap_profile: Option<String>,
     #[serde(default)]
     pub keymap_path: Option<String>,
 }
@@ -1654,6 +1656,10 @@ impl UiSettings {
         IconMode::Auto
     }
 
+    fn default_keymap_profile() -> Option<String> {
+        None
+    }
+
     fn deserialize_role_label_mode<'de, D>(
         deserializer: D,
     ) -> std::result::Result<RoleLabelDisplay, D::Error>
@@ -1723,6 +1729,7 @@ impl Default for UiSettings {
             render_markdown: Self::default_render_markdown(),
             show_timestamps: Self::default_show_timestamps(),
             icon_mode: Self::default_icon_mode(),
+            keymap_profile: Self::default_keymap_profile(),
             keymap_path: None,
         }
     }