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/components/sidebar/Sidebar.tsx

@@ -7,6 +7,7 @@ import { AccountSwitcher } from "./AccountSwitcher";
 const NAV_ITEMS = [
   { id: "feed" as const, label: "feed", icon: "◈" },
   { id: "search" as const, label: "search", icon: "⌕" },
+  { id: "dm" as const, label: "messages", icon: "✉" },
   { id: "zaps" as const, label: "zaps", icon: "⚡" },
   { id: "relays" as const, label: "relays", icon: "⟐" },
   { id: "settings" as const, label: "settings", icon: "⚙" },