Read config.toml, keys.toml and themes, and open them with :config (stage 8b)

The config folder now sets everything: config.toml the settings (now
also `theme` and `editor`), keys.toml the keys, and themes/<name>.toml
the theme, laid over the built-in one, so a theme only needs what it
changes. Each file is applied again a second after it's saved; a
mistake only drops its own line, and the status bar says what's wrong,
naming the file.

:config lists the three files with their folder; Enter hands the
terminal to your editor (the editor setting, $VISUAL, $EDITOR, else
notepad or vi), writing the file first if it's missing, with every
option at its default behind a #, explained (keys.toml lists every
command by name). :theme switches themes until cumulus quits, like
:set.

The built-in theme gets the named styles the user picked, each also put
on by its tag: question (yellow, dashed), important (red, thick) and done
(gray, its title faded); and link styles blocks and supports, whose
label shows on a link without its own. Themes can also set [modes],
[node], [root], [branch] and [link].

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/action.rs

@@ -10,6 +10,7 @@ use super::panes::Arrange;
 use super::popup::{MenuFor, PickKind};
 use super::text_edit::StartEdit;
 use crate::command::parse::SetArg;
+use crate::config::ConfigFile;
 use crate::input::command::Command;
 use crate::input::key::Key;
 use crate::model::{Direction, EditOp, Group, NodeId, Title};
@@ -157,6 +158,10 @@ pub enum AppAction {
     SwitchDocument(DocumentChoice),
 
     // Files.
+    /// `:config`: the config files, to open one in the editor.
+    OpenConfig,
+    /// Open a config file in the editor (made first if it isn't there).
+    EditConfig(ConfigFile),
     /// Save (`:w`, `:x`, `:wa`).
     Save(SaveWhich),
     /// Open a document in the focused pane (`:e`), from a path as typed: