Add Direct Messages — NIP-04 encrypted DMs (roadmap #13)

Nostr layer:
- fetchDMConversations: fetches all kind-4 events to/from the user
  (both directions in parallel), deduplicates, newest-first
- fetchDMThread: fetches both directions for a specific conversation,
  sorted oldest-first for display
- sendDM: NIP-04 encrypts content via NDK signer, publishes kind 4
- decryptDM: decrypts regardless of direction (ECDH shared secret is
  symmetric — always pass the other party to signer.decrypt)

UI (DMView):
- Two-panel layout: conversation list (w-56) + active thread
- ConvRow: avatar, name, time; shows "🔒 encrypted" preview to avoid
  decrypting the whole inbox on load
- MessageBubble: decrypts lazily on mount; mine right-aligned,
  theirs left-aligned; shows "Could not decrypt" on failure
- ThreadPanel: loads full thread, auto-scrolls to bottom, re-fetches
  after send; Ctrl+Enter to send
- NewConvInput: start a new conversation by pasting an npub1 or hex
  pubkey; validates and resolves before opening thread
- Read-only (npub) accounts see a clear "nsec required" message

Navigation:
- ✉ messages added to sidebar nav
- openDM(pubkey) in UI store → navigates to dm view with pending pubkey
- ProfileView: "✉ message" button in action row opens DM thread

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

src/stores/ui.ts

@@ -2,7 +2,7 @@ import { create } from "zustand";
 
 import { NDKEvent } from "@nostr-dev-kit/ndk";
 
-type View = "feed" | "search" | "relays" | "settings" | "profile" | "thread" | "article-editor" | "about" | "zaps";
+type View = "feed" | "search" | "relays" | "settings" | "profile" | "thread" | "article-editor" | "about" | "zaps" | "dm";
 
 interface UIState {
   currentView: View;
@@ -11,10 +11,12 @@ interface UIState {
   selectedNote: NDKEvent | null;
   previousView: View;
   pendingSearch: string | null;
+  pendingDMPubkey: string | null;
   setView: (view: View) => void;
   openProfile: (pubkey: string) => void;
   openThread: (note: NDKEvent, from: View) => void;
   openSearch: (query: string) => void;
+  openDM: (pubkey: string) => void;
   goBack: () => void;
   toggleSidebar: () => void;
 }
@@ -28,10 +30,12 @@ export const useUIStore = create<UIState>((set, _get) => ({
   selectedNote: null,
   previousView: "feed",
   pendingSearch: null,
+  pendingDMPubkey: null,
   setView: (currentView) => set({ currentView }),
   openProfile: (pubkey) => set((s) => ({ currentView: "profile", selectedPubkey: pubkey, previousView: s.currentView as View })),
   openThread: (note, from) => set({ currentView: "thread", selectedNote: note, previousView: from }),
   openSearch: (query) => set({ currentView: "search", pendingSearch: query }),
+  openDM: (pubkey) => set({ currentView: "dm", pendingDMPubkey: pubkey }),
   goBack: () => set((s) => ({
     currentView: s.previousView !== s.currentView ? s.previousView : "feed",
     selectedNote: null,