feat: add theme selector with multiple dark themes

Add theme dropdown in settings with Light, Dark, System plus
Zinc, Midnight, Claude, and Ghostty dark variants. Each theme
has its own accent color and surface tints. Desaturate terminal
ANSI colors across all dark themes. Add surfaceSidebarHover
semantic token so hover states go the right direction in both
light and dark modes. Self-heal if a stored theme no longer exists.

packages/app/src/components/explorer-sidebar.tsx

@@ -472,7 +472,7 @@ const styles = StyleSheet.create((theme) => ({
     borderRadius: theme.borderRadius.md,
   },
   tabActive: {
-    backgroundColor: theme.colors.surface1,
+    backgroundColor: theme.colors.surfaceSidebarHover,
   },
   tabText: {
     fontSize: theme.fontSize.sm,

packages/app/src/components/file-explorer-pane.tsx

@@ -849,7 +849,7 @@ const styles = StyleSheet.create((theme) => ({
     borderRadius: theme.borderRadius.md,
   },
   entryRowActive: {
-    backgroundColor: theme.colors.surface1,
+    backgroundColor: theme.colors.surfaceSidebarHover,
   },
   indentGuide: {
     position: "absolute",

packages/app/src/components/left-sidebar.tsx

@@ -885,10 +885,10 @@ const styles = StyleSheet.create((theme) => ({
     color: theme.colors.foreground,
   },
   newAgentButtonHovered: {
-    backgroundColor: theme.colors.surface1,
+    backgroundColor: theme.colors.surfaceSidebarHover,
   },
   newAgentButtonActive: {
-    backgroundColor: theme.colors.surface1,
+    backgroundColor: theme.colors.surfaceSidebarHover,
   },
   hostTrigger: {
     flexDirection: "row",
@@ -901,7 +901,7 @@ const styles = StyleSheet.create((theme) => ({
     borderRadius: theme.borderRadius.lg,
   },
   hostTriggerHovered: {
-    backgroundColor: theme.colors.surface1,
+    backgroundColor: theme.colors.surfaceSidebarHover,
   },
   hostStatusDot: {
     width: 8,

packages/app/src/components/sidebar-workspace-list.tsx

@@ -1980,7 +1980,7 @@ const styles = StyleSheet.create((theme) => ({
     gap: theme.spacing[2],
   },
   projectRowHovered: {
-    backgroundColor: theme.colors.surface1,
+    backgroundColor: theme.colors.surfaceSidebarHover,
   },
   projectRowPressed: {
     backgroundColor: theme.colors.surface2,
@@ -2050,7 +2050,7 @@ const styles = StyleSheet.create((theme) => ({
     flexShrink: 0,
   },
   projectActionButtonHovered: {
-    backgroundColor: theme.colors.surface1,
+    backgroundColor: theme.colors.surfaceSidebarHover,
   },
   projectActionButtonText: {
     color: theme.colors.foregroundMuted,
@@ -2065,7 +2065,7 @@ const styles = StyleSheet.create((theme) => ({
     flexShrink: 0,
   },
   projectIconActionButtonHovered: {
-    backgroundColor: theme.colors.surface1,
+    backgroundColor: theme.colors.surfaceSidebarHover,
   },
   projectIconActionButtonHidden: {
     opacity: 0,
@@ -2143,7 +2143,7 @@ const styles = StyleSheet.create((theme) => ({
     flexShrink: 0,
   },
   workspaceRowHovered: {
-    backgroundColor: theme.colors.surface1,
+    backgroundColor: theme.colors.surfaceSidebarHover,
   },
   workspaceRowPressed: {
     backgroundColor: theme.colors.surface2,
@@ -2157,7 +2157,7 @@ const styles = StyleSheet.create((theme) => ({
     ...theme.shadow.md,
   },
   sidebarRowSelected: {
-    backgroundColor: theme.colors.surface1,
+    backgroundColor: theme.colors.surfaceSidebarHover,
   },
   workspaceRowContainer: {
     position: "relative",