feat(panel): island, rail by day, companion mode beside a deep note, cached index and threads, obsidian skin for strips

src/state.svelte.ts

@@ -5,4 +5,6 @@ export class PanelState {
   selected = $state<string | null>(null);
   follow = $state(true);
   sourcePath = $state("");
+  // The selected conversation is the open note's own: show activity, not a copy.
+  companion = $state(false);
 }