feat: Remember last selected DM conversation

Add localStorage persistence for Direct Messages page to remember and restore the last selected conversation when user returns to the page.

Changes:
- Save selected conversation to localStorage when user selects a conversation
- Restore last conversation on page load (if no URL parameter provided)
- Clear localStorage when user returns to empty state
- Updated README.md with persistence documentation
- Priority: URL parameter > localStorage > empty state

This improves UX by maintaining conversation context across page navigation, similar to how the main page remembers the selected channel.

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

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

README.md

@@ -355,6 +355,11 @@ Access the Direct Messages feature:
 4. Press Enter or click Send
 5. Click "Back" button to return to the main chat view
 
+**Persistence:**
+- The app remembers your last selected conversation
+- When you return to the DM page, it automatically opens the last conversation you were viewing
+- This works similarly to how the main page remembers your selected channel
+
 **Note:** Only client contacts (CLI) are shown in the dropdown. Repeaters (REP), rooms (ROOM), and sensors (SENS) are automatically filtered out.
 
 **Message status indicators:**

app/static/js/dm.js

@@ -39,11 +39,17 @@ document.addEventListener('DOMContentLoaded', async function() {
     // Load connection status
     await loadStatus();
 
-    // Check for initial conversation from URL parameter
+    // Check for initial conversation from URL parameter, or restore last active conversation
     if (window.MC_CONFIG && window.MC_CONFIG.initialConversation) {
         const convId = window.MC_CONFIG.initialConversation;
         // Find the conversation in the list or use the ID directly
         selectConversation(convId);
+    } else {
+        // Restore last selected conversation from localStorage
+        const savedConversation = localStorage.getItem('mc_active_dm_conversation');
+        if (savedConversation) {
+            selectConversation(savedConversation);
+        }
     }
 
     // Setup auto-refresh
@@ -244,6 +250,9 @@ function populateConversationSelector() {
 async function selectConversation(conversationId) {
     currentConversationId = conversationId;
 
+    // Save to localStorage for next visit
+    localStorage.setItem('mc_active_dm_conversation', conversationId);
+
     // Find the conversation to get recipient name
     const conv = dmConversations.find(c => c.conversation_id === conversationId);
     if (conv) {
@@ -287,6 +296,9 @@ function clearConversation() {
     currentConversationId = null;
     currentRecipient = null;
 
+    // Clear from localStorage
+    localStorage.removeItem('mc_active_dm_conversation');
+
     // Disable input
     const input = document.getElementById('dmMessageInput');
     const sendBtn = document.getElementById('dmSendBtn');