feat(settings): configurable selection marker, default off

Adds a Selection Marker toggle under Settings → Preferences that
controls the ❯ cursor glyph on the focused row of every list.
Default off; rows keep a leading space for alignment either way.
Every list pane (tab strip, feed, my shows, discover, search,
settings, whitelist editor) reads the glyph through the shared
useSelectionMarker hook.

Also aligns the lead column across panes: page list rows drop
their extra box paddingLeft so labels start at the same column
as the tab strip (2 cells without nerd icons, 4 with), keeping
episode sub-rows aligned to the title.

src/hooks/useSelectionMarker.ts

@@ -0,0 +1,16 @@
+/**
+ * useSelectionMarker — reactive accessor for the row-selection marker glyph.
+ *
+ * When the `showSelectionMarker` setting is on, the focused row of every list
+ * renders `❯`; when off (the default), it renders a space so column alignment
+ * is preserved. Every list pane in the app reads the marker through this hook
+ * so the setting applies consistently everywhere.
+ */
+
+import { useAppStore } from "@/stores/app";
+
+export function useSelectionMarker(): () => string {
+	const app = useAppStore();
+	return () =>
+		app.state().settings.showSelectionMarker ? "❯" : " ";
+}