fix(messages): queue-based sending + simple dedup

Root cause: rapid Enter sends caused parallel API calls creating
real duplicates in DB. Fix: message queue processes one at a time.

- sendContent() adds to queue + shows optimistic UI instantly
- _processQueue() sends sequentially (await each before next)
- Polling skips own messages (they come via optimistic UI)
- Simple ID-only dedup in appendMessage (no complex content matching)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/js/conversations.js

@@ -727,22 +727,8 @@
             if (!state.messages[convId]) state.messages[convId] = [];
 
             // Dedup: skip if message with this ID already exists
-            // Dedup: skip if exact ID match OR if optimistic message from same sender within 10s
-            var dominated = state.messages[convId].some(function(m) {
-                if (m.id === msg.id) return true;
-                if (m._optimistic && msg.sender_id && m.sender_id === msg.sender_id) {
-                    // Time-based match: if optimistic msg was sent within 10s, it's the same
-                    var mTime = new Date(m.created_at).getTime();
-                    var msgTime = new Date(msg.created_at).getTime();
-                    if (Math.abs(mTime - msgTime) < 10000) {
-                        m.id = msg.id;
-                        m._optimistic = false;
-                        return true;
-                    }
-                }
-                return false;
-            });
-            if (dominated) return;
+            // Simple dedup by ID (string or number)
+            if (msg.id && state.messages[convId].some(function(m) { return m.id === msg.id; })) return;
             state.messages[convId].push(msg);
 
             if (convId !== state.currentConversationId) return;
@@ -1288,21 +1274,48 @@
             }
         },
 
-        // sendContent: called with pre-captured content (from Enter keydown)
-        sendContent: async function (html, text) {
-            if (!state.currentConversationId) return;
-            var convId = state.currentConversationId;
-            var savedReplyTo = state.replyToMessage;
-            var savedFiles = state.attachedFiles.slice();
+        // Queue-based sending: Enter adds to queue, queue processes one at a time
+        _sendQueue: [],
+        _queueProcessing: false,
 
-            // Clear state (editor already cleared by keydown handler)
+        sendContent: function (html, text) {
+            if (!state.currentConversationId) return;
+            // Add to queue with current state snapshot
+            Composer._sendQueue.push({
+                convId: state.currentConversationId,
+                html: html,
+                text: text,
+                replyTo: state.replyToMessage,
+                files: state.attachedFiles.slice()
+            });
             state.attachedFiles = [];
             state.replyToMessage = null;
             var replyPreview = document.getElementById('replyPreview');
             if (replyPreview) replyPreview.style.display = 'none';
             Composer.renderAttachments();
+            // Show optimistic message immediately
+            var tempId = 'temp-' + Date.now() + '-' + Math.random();
+            ChatView.appendMessage({
+                id: tempId,
+                conversation_id: state.currentConversationId,
+                content: html,
+                sender_id: window.__CURRENT_USER__ ? window.__CURRENT_USER__.id : null,
+                sender: window.__CURRENT_USER__ || {},
+                created_at: new Date().toISOString(),
+                _optimistic: true
+            });
+            // Process queue
+            Composer._processQueue();
+        },
 
-            return Composer._doSend(convId, html, text, savedReplyTo, savedFiles);
+        _processQueue: async function () {
+            if (Composer._queueProcessing || !Composer._sendQueue.length) return;
+            Composer._queueProcessing = true;
+            while (Composer._sendQueue.length > 0) {
+                var item = Composer._sendQueue.shift();
+                await Composer._doSend(item.convId, item.html, item.text, item.replyTo, item.files);
+            }
+            Composer._queueProcessing = false;
         },
 
         // send: called from button click — reads from Quill
@@ -1329,20 +1342,7 @@
         },
 
         _doSend: async function (convId, html, text, savedReplyTo, savedFiles) {
-
-            // Show optimistic message in DOM instantly
-            var tempId = 'temp-' + Date.now();
-            var optimisticMsg = {
-                id: tempId,
-                conversation_id: convId,
-                content: html,
-                sender_id: window.__CURRENT_USER__ ? window.__CURRENT_USER__.id : null,
-                sender: window.__CURRENT_USER__ || {},
-                created_at: new Date().toISOString(),
-                _optimistic: true
-            };
-            ChatView.appendMessage(optimisticMsg);
-
+            // Optimistic message already shown by sendContent() — just do the API call
             try {
                 var fd = new FormData();
                 if (html && text) fd.append('content', html);
@@ -1714,8 +1714,12 @@
                         if (!data.messages || !data.messages.length) return;
 
                         // Find messages newer than what we have
+                        var currentUserId = window.__CURRENT_USER__ ? window.__CURRENT_USER__.id : null;
                         var newMsgs = data.messages.filter(function (msg) {
-                            return msg.id > newestId;
+                            if (msg.id <= newestId) return false;
+                            // Skip own messages — already displayed by optimistic UI
+                            if (currentUserId && msg.sender_id === currentUserId) return false;
+                            return true;
                         });
 
                         if (newMsgs.length > 0) {