Make topbar moon button actually toggle theme

The Moon icon previously navigated to ely://settings/appearance,
which is misleading for a button visually framed as a one-tap theme
control. Add cycle_theme_mode (System → Light → Dark → System) and
swap the icon between Sun and Moon to mirror the active state.

crates/ely_app/src/shell/chrome/topbar.rs

@@ -1,6 +1,6 @@
 use ely_browser_core::BrowserSnapshot;
 use ely_design_system::{colors, spacing};
-use ely_domain::BrowserTab;
+use ely_domain::{BrowserTab, ThemeMode};
 use gpui::{
     AnyElement, BoxShadow, Context, FontWeight, InteractiveElement, IntoElement, ParentElement,
     SharedString, StatefulInteractiveElement, Styled, div, hsla, point,
@@ -47,9 +47,9 @@ pub(crate) fn render_topbar(
         ))
         .child(render_topbar_action(
             "toggle-theme",
-            IconName::Moon,
+            theme_mode_icon(snapshot.appearance.theme_mode()),
             cx,
-            |shell, window, cx| shell.open_internal_tab("ely://settings/appearance", window, cx),
+            |shell, _window, cx| shell.cycle_theme_mode(cx),
         ))
         .child(render_topbar_action(
             "open-menu",
@@ -249,6 +249,18 @@ where
 const OMNIBAR_BG: u32 = 0xffffff8c;
 const CHIP_HOVER_BG: u32 = 0xffffffd9;
 
+/// Topbar quick-toggle icon for the current theme mode. The button
+/// cycles System → Light → Dark → System, and the icon previews the
+/// state the user is in: sun for light, moon for dark. System falls
+/// back to moon since the bundled icon set has no combined sun-moon
+/// glyph and the OS-driven mode visually leans neutral.
+fn theme_mode_icon(mode: ThemeMode) -> IconName {
+    match mode {
+        ThemeMode::Light => IconName::Sun,
+        ThemeMode::System | ThemeMode::Dark => IconName::Moon,
+    }
+}
+
 fn soft_shadow() -> Vec<BoxShadow> {
     vec![
         BoxShadow {

crates/ely_app/src/shell/settings_actions.rs

@@ -72,6 +72,23 @@ impl ElyShell {
         }
     }
 
+    /// Cycle the theme mode for the topbar's quick-toggle button:
+    /// System → Light → Dark → System. Mirrors the segmented control
+    /// in the appearance settings page so the topbar toggle reaches
+    /// every state without spawning a settings page.
+    pub(super) fn cycle_theme_mode(&mut self, cx: &mut Context<Self>) {
+        let ShellState::Ready(core) = &mut self.state else {
+            return;
+        };
+        let next = match core.appearance().theme_mode() {
+            ThemeMode::System => ThemeMode::Light,
+            ThemeMode::Light => ThemeMode::Dark,
+            ThemeMode::Dark => ThemeMode::System,
+        };
+        core.set_theme_mode(next);
+        cx.notify();
+    }
+
     pub(super) fn toggle_reduce_motion(&mut self, cx: &mut Context<Self>) {
         if let ShellState::Ready(core) = &mut self.state {
             let next = !core.appearance().reduce_motion();