Add thread view with replies

- Click note content to open thread view
- ThreadView shows root note, reply composer, and replies
- fetchReplies added to nostr lib (kind 1 #e filter)
- UI store gains openThread, goBack, previousView for navigation history
- Profile back button now returns to previous view correctly

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/stores/ui.ts

@@ -1,21 +1,31 @@
 import { create } from "zustand";
 
-type View = "feed" | "relays" | "settings" | "profile";
+import { NDKEvent } from "@nostr-dev-kit/ndk";
+
+type View = "feed" | "relays" | "settings" | "profile" | "thread";
 
 interface UIState {
   currentView: View;
   sidebarCollapsed: boolean;
   selectedPubkey: string | null;
+  selectedNote: NDKEvent | null;
+  previousView: View;
   setView: (view: View) => void;
   openProfile: (pubkey: string) => void;
+  openThread: (note: NDKEvent, from: View) => void;
+  goBack: () => void;
   toggleSidebar: () => void;
 }
 
-export const useUIStore = create<UIState>((set) => ({
+export const useUIStore = create<UIState>((set, get) => ({
   currentView: "feed",
   sidebarCollapsed: false,
   selectedPubkey: null,
+  selectedNote: null,
+  previousView: "feed",
   setView: (currentView) => set({ currentView }),
-  openProfile: (pubkey) => set({ currentView: "profile", selectedPubkey: pubkey }),
+  openProfile: (pubkey) => set((s) => ({ currentView: "profile", selectedPubkey: pubkey, previousView: s.currentView as View })),
+  openThread: (note, from) => set({ currentView: "thread", selectedNote: note, previousView: from }),
+  goBack: () => set((s) => ({ currentView: s.previousView, selectedNote: null })),
   toggleSidebar: () => set((s) => ({ sidebarCollapsed: !s.sidebarCollapsed })),
 }));