Rebuild /settings/appearance as the design form

The appearance route now opens with the design layout: serif "GENERAL"
overline + "Appearance" headline + intro paragraph, a four-tile
wallpaper picker (Dawn/Violet/Mint/Slate with active outline + check
glyph), a theme-mode segmented control, an accent swatch row, a
reduce-motion toggle, and a reset row. Every control mutates real state
through new shell methods (set_wallpaper_theme, set_theme_mode,
toggle_reduce_motion, reset_appearance) which delegate to the core.

The chrome lives in chrome::appearance_form so the page module is a
six-line shim. Light/Dark theme buttons currently switch the persisted
mode; rendering swaps will land when light/dark token sheets ship —
keeping the persistence so the eventual flip is one place.

crates/ely_app/src/shell/settings_actions.rs

@@ -1,7 +1,7 @@
 use ely_domain::{
     ArchivePolicy, DiagnosticsReportingPolicy, DownloadPolicy, FavoriteLimit,
     HistoryRecordingPolicy, NewTabDestination, ProfileId, ProfileSyncPolicy, SearchEngine,
-    SyncObjectKind, SyncObjectPolicy, UpdatePolicy,
+    SyncObjectKind, SyncObjectPolicy, ThemeMode, UpdatePolicy, WallpaperTheme,
 };
 use gpui::Context;
 
@@ -49,6 +49,43 @@ impl ElyShell {
         }
     }
 
+    pub(super) fn set_wallpaper_theme(
+        &mut self,
+        wallpaper: WallpaperTheme,
+        cx: &mut Context<Self>,
+    ) {
+        if let ShellState::Ready(core) = &mut self.state {
+            core.set_wallpaper_theme(wallpaper);
+            cx.notify();
+        }
+    }
+
+    pub(super) fn set_theme_mode(
+        &mut self,
+        theme_mode: ThemeMode,
+        cx: &mut Context<Self>,
+    ) {
+        if let ShellState::Ready(core) = &mut self.state {
+            core.set_theme_mode(theme_mode);
+            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();
+            core.set_reduce_motion(next);
+            cx.notify();
+        }
+    }
+
+    pub(super) fn reset_appearance(&mut self, cx: &mut Context<Self>) {
+        if let ShellState::Ready(core) = &mut self.state {
+            core.reset_appearance();
+            cx.notify();
+        }
+    }
+
     pub(super) fn reset_general_settings(&mut self, cx: &mut Context<Self>) {
         if let ShellState::Ready(core) = &mut self.state {
             core.reset_general_settings();