feat(chat): require explicit send button on touch devices

Enter on a phone's virtual keyboard now inserts a newline instead of
sending, in both group chat and DM. Detected via the pointer:coarse
media query - no user-facing setting, since accidental sends from a
mistapped Enter were the actual complaint. Desktop keeps Enter-to-send.

app/static/js/app.js

@@ -636,9 +636,11 @@ function setupEventListeners() {
         sendMessage();
     });
 
-    // Handle Enter key (send) vs Shift+Enter (new line)
+    // Handle Enter key (send) vs Shift+Enter (new line) - desktop only.
+    // Touch devices send via the button so an accidental Enter on the virtual
+    // keyboard doesn't fire a half-typed message.
     input.addEventListener('keydown', function(e) {
-        if (e.key === 'Enter' && !e.shiftKey) {
+        if (e.key === 'Enter' && !e.shiftKey && !isTouchPrimaryDevice()) {
             e.preventDefault();
             sendMessage();
         }

app/static/js/dm.js

@@ -437,9 +437,10 @@ function setupEventListeners() {
     if (input) {
         input.addEventListener('input', updateCharCounter);
 
-        // Enter key to send
+        // Enter key to send (desktop only - touch devices send via the button
+        // so an accidental Enter on the virtual keyboard doesn't fire a half-typed message)
         input.addEventListener('keydown', function(e) {
-            if (e.key === 'Enter' && !e.shiftKey) {
+            if (e.key === 'Enter' && !e.shiftKey && !isTouchPrimaryDevice()) {
                 e.preventDefault();
                 sendMessage();
             }

app/static/js/message-utils.js

@@ -3,6 +3,16 @@
  * Handles mention badges, URL links, and image previews
  */
 
+/**
+ * Detect whether the primary input is touch-based (phones/tablets), as opposed
+ * to a device with a precise pointer (mouse/trackpad). Used to decide whether
+ * Enter should send a message or just insert a newline.
+ * @returns {boolean}
+ */
+function isTouchPrimaryDevice() {
+    return window.matchMedia('(pointer: coarse)').matches;
+}
+
 /**
  * Process message content to handle mentions, URLs, and images
  * @param {string} content - Raw message content