test: keep the settings page counting the sections it has

`docs/customization/settings.mdx` says "The eight sections" and then
shows one card each. `SettingsSection::ALL` has eight, and the names
line up — Appearance, Terminal, Input, SSH, Agents, Window & Tabs,
Keybindings, About.

Adding a ninth means editing an array in `settings.rs` and a page in
`docs/` that nothing links to it. The count is stated twice there, in
words and in cards, so a section added without touching the page leaves
two wrong statements on the page that explains the window.

Both are now read by the test, the same move as the READMEs' agent count
in the last commit. Not vacuous: the page has no "The nine sections" and
exactly eight cards, so a ninth section fails on both assertions.

src/ui/settings.rs

@@ -7188,6 +7188,36 @@ impl Tty7App {
 mod tests {
     use super::*;
 
+    /// The settings page documentation counts the sections, twice.
+    ///
+    /// A heading in words and one card per section, in a file that is not
+    /// opened when a section is added to `SettingsSection::ALL`. Same guard as
+    /// the READMEs' agent count: read the other file, so a ninth section is a
+    /// failing test rather than a page that quietly says eight.
+    #[test]
+    fn the_settings_page_documents_every_section() {
+        const DOC: &str = include_str!("../../docs/customization/settings.mdx");
+        let n = SettingsSection::ALL.len();
+
+        let word = [
+            "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten",
+            "eleven", "twelve",
+        ]
+        .get(n)
+        .copied()
+        .unwrap_or("many");
+        assert!(
+            DOC.contains(&format!("The {word} sections")),
+            "the page does not say it has {word} ({n}) sections"
+        );
+
+        let cards = DOC.matches("<Card title=").count();
+        assert_eq!(
+            cards, n,
+            "the page shows {cards} section cards for {n} sections"
+        );
+    }
+
     /// A shortcut is the first thing someone searching a settings window for a
     /// feature by name is after, and the Keybindings page was the one page the
     /// search could not see into: searching "split" found the settings that