Remove duplicate "grey" theme, keep "dark" (ticket 004)

"grey" and "dark" rendered identically apart from accent colour, so
drop "grey" as a selectable colour scheme everywhere in the frontend
(index.css, api.ts's ColorScheme type, the NavMenu picker, the
downloadable BUILTIN_THEMES table) and keep "dark" as the single
equivalent theme.

Existing settings.json files saved with color_scheme: "grey" keep
loading correctly: the backend ColorScheme enum aliases the legacy
"grey" serde value onto Dark, so old configs transparently migrate to
"dark" without any manual intervention.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/settings.rs

@@ -18,12 +18,17 @@ use crate::alerts::AlertRule;
 pub type SettingsHandle = Arc<Mutex<Settings>>;
 
 /// User's preferred colour scheme for the webapp.
+///
+/// `Dark` accepts the legacy `"grey"` serde value (ticket 004: "grey" was a
+/// near-duplicate of "dark" and has been removed as a selectable option) so a
+/// settings file saved before that change still loads cleanly instead of
+/// falling back to `System`.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
 #[serde(rename_all = "lowercase")]
 pub enum ColorScheme {
     Light,
+    #[serde(alias = "grey")]
     Dark,
-    Grey,
     Red,
     Green,
     Blue,
@@ -316,6 +321,15 @@ mod tests {
         assert_eq!(reloaded, settings);
     }
 
+    #[test]
+    fn legacy_grey_color_scheme_deserializes_as_dark() {
+        let settings: Settings = serde_json::from_str(
+            r#"{"color_scheme":"grey","css_entries":[],"custom_theme_id":null,"profiles":{"names":[],"active_profile":null,"profiles_dir":"profiles"},"max_logs":null,"alert_rules":[],"ntfy_topic":null,"ntfy_server":null}"#,
+        )
+        .unwrap();
+        assert_eq!(settings.color_scheme, ColorScheme::Dark);
+    }
+
     #[test]
     fn round_trips_through_save_and_load() {
         let dir = tempfile::tempdir().unwrap();