Add DM inbox with NIP-04/NIP-17 support and soft delete

Features:
- Full DM inbox UI with conversation list and message view
- Support for both NIP-04 (kind 4) and NIP-17 (kind 14/1059) encryption
- Progressive message decryption with background loading
- Soft delete using kind 30078 Application Specific Data events
- Message selection UI with delete selected/delete all
- Undelete all functionality per conversation
- Jump to newest button with new message counter
- Conversation filtering (all / follows only)
- Per-conversation relay and encryption settings
- New messages indicator on sidebar (clears when inbox viewed)
- Follow indicator on conversation items

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/components/Sidebar/InboxButton.tsx

@@ -0,0 +1,25 @@
+import { usePrimaryPage } from '@/PageManager'
+import { useDM } from '@/providers/DMProvider'
+import { MessageSquare } from 'lucide-react'
+import SidebarItem from './SidebarItem'
+
+export default function InboxButton({ collapse }: { collapse: boolean }) {
+  const { navigate, current, display } = usePrimaryPage()
+  const { hasNewMessages } = useDM()
+
+  return (
+    <SidebarItem
+      title="Inbox"
+      onClick={() => navigate('inbox')}
+      active={display && current === 'inbox'}
+      collapse={collapse}
+    >
+      <div className="relative">
+        <MessageSquare />
+        {hasNewMessages && (
+          <div className="absolute -top-1 right-0 w-2 h-2 ring-2 ring-background bg-primary rounded-full" />
+        )}
+      </div>
+    </SidebarItem>
+  )
+}